[PATCH v4 0/5] block: fix blktrace debugfs use after free

[PATCH v4 0/5] block: fix blktrace debugfs use after free

[Luis Chamberlain]

Phew, well, since we did't hear back about removing scsi-generic
blktrace functionality I put work into addressing to keep it. That
took a lot of code inspection and also testing. Since libvirt
is limited to what devices you can test I resorted to testing
all supported device types with iscsi tcp and tgt.

I decided to simplfiy the partition work further by just using
a symbolic link. In the end that makes the blktrace code even
cleaner than anything we had before.

scsi-generic stuff still required quite a bit of work to figure
out what to do. Since scsi devices probe asynchronously and scsi-generic
is nothing but a class_interface whose sg_add_device() runs *prior*
to the scsi device probe, we currently address the symlink on the
sg ioctl. I however think this reveals a shortcoming of the class
interface, now that we have async probe and its used widely. I
think we need a probe_complete() call or something like that.
If that seems reasonable I can work on that, that would allow us to
move the debugfs_dir symlink / settings from sg's ioctl to a new
proper call. I'd prefer to address that later though, as an evolution.

Its my first time touching scsi stuff, so I'd highly appreciate a good
review of what I propose for scsi-generic. It gets a bit more
complicated with some drivers using the bsg queue. FWIW, if bsg is
enabled we *reshare* the request_queue from the scsi_device, unless
you're a scsi transport, in which case it creates its own.

You can find this on my git tree:

https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux-next.git/log/?h=20200508-block-fixes

Luis Chamberlain (5):
  block: revert back to synchronous request_queue removal
  block: move main block debugfs initialization to its own file
  blktrace: fix debugfs use after free
  blktrace: break out of blktrace setup on concurrent calls
  loop: be paranoid on exit and prevent new additions / removals

 block/Makefile               |   1 +
 block/blk-core.c             |  32 ++++--
 block/blk-debugfs.c          | 202 +++++++++++++++++++++++++++++++++++
 block/blk-mq-debugfs.c       |   5 -
 block/blk-sysfs.c            |  46 ++++----
 block/blk.h                  |  23 ++++
 block/bsg.c                  |   2 +
 block/genhd.c                |  73 ++++++++++++-
 block/partitions/core.c      |   9 ++
 drivers/block/loop.c         |   4 +
 drivers/scsi/ch.c            |   1 +
 drivers/scsi/sg.c            |  75 +++++++++++++
 drivers/scsi/st.c            |   2 +
 include/linux/blkdev.h       |   6 +-
 include/linux/blktrace_api.h |   1 -
 include/linux/genhd.h        |  69 ++++++++++++
 kernel/trace/blktrace.c      |  33 ++++--
 17 files changed, 539 insertions(+), 45 deletions(-)
 create mode 100644 block/blk-debugfs.c

-- 
2.25.1

[PATCH v4 1/5] block: revert back to synchronous request_queue removal

[Luis Chamberlain]

Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
v4.12 moved the work behind blk_release_queue() into a workqueue after a
splat floated around which indicated some work on blk_release_queue()
could sleep in blk_exit_rl(). This splat would be possible when a driver
called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
as its final call) from an atomic context.

blk_put_queue() decrements the refcount for the request_queue kobject,
and upon reaching 0 blk_release_queue() is called. Although blk_exit_rl()
is now removed through commit db6d9952356 ("block: remove request_list code")
on v5.0, we reserve the right to be able to sleep within blk_release_queue()
context.

The last reference for the request_queue must not be called from atomic
context. *When* the last reference to the request_queue reaches 0 varies,
and so let's take the opportunity to document when that is expected to
happen and also document the context of the related calls as best as possible
so we can avoid future issues, and with the hopes that the synchronous
request_queue removal sticks.

We revert back to synchronous request_queue removal because asynchronous
removal creates a regression with expected userspace interaction with
several drivers. An example is when removing the loopback driver, one
uses ioctls from userspace to do so, but upon return and if successful,
one expects the device to be removed. Likewise if one races to add another
device the new one may not be added as it is still being removed. This was
expected behavior before and it now fails as the device is still present
and busy still. Moving to asynchronous request_queue removal could have
broken many scripts which relied on the removal to have been completed if
there was no error. Document this expectation as well so that this
doesn't regress userspace again.

Using asynchronous request_queue removal however has helped us find
other bugs. In the future we can test what could break with this
arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: yu kuai <yukuai3@huawei.com>
Suggested-by: Nicolai Stange <nstange@suse.de>
Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 block/blk-core.c       | 23 +++++++++++++
 block/blk-sysfs.c      | 43 +++++++++++++------------
 block/genhd.c          | 73 +++++++++++++++++++++++++++++++++++++++++-
 include/linux/blkdev.h |  2 --
 4 files changed, 117 insertions(+), 24 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index dccdae09b7b6..da120fd257fa 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -306,6 +306,16 @@ void blk_clear_pm_only(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_clear_pm_only);
 
+/**
+ * blk_put_queue - decrement the request_queue refcount
+ * @q: the request_queue structure to decrement the refcount for
+ *
+ * Decrements the refcount of the request_queue kobject. When this reaches 0
+ * we'll have blk_release_queue() called.
+ *
+ * Context: Any context, but the last reference must not be dropped from
+ *          atomic context.
+ */
 void blk_put_queue(struct request_queue *q)
 {
 	kobject_put(&q->kobj);
@@ -337,9 +347,14 @@ EXPORT_SYMBOL_GPL(blk_set_queue_dying);
  *
  * Mark @q DYING, drain all pending requests, mark @q DEAD, destroy and
  * put it.  All future requests will be failed immediately with -ENODEV.
+ *
+ * Context: can sleep
  */
 void blk_cleanup_queue(struct request_queue *q)
 {
+	/* cannot be called from atomic context */
+	might_sleep();
+
 	WARN_ON_ONCE(blk_queue_registered(q));
 
 	/* mark @q DYING, no new request or merges will be allowed afterwards */
@@ -584,6 +599,14 @@ struct request_queue *blk_alloc_queue(make_request_fn make_request, int node_id)
 }
 EXPORT_SYMBOL(blk_alloc_queue);
 
+/**
+ * blk_get_queue - increment the request_queue refcount
+ * @q: the request_queue structure to increment the refcount for
+ *
+ * Increment the refcount of the request_queue kobject.
+ *
+ * Context: Any context.
+ */
 bool blk_get_queue(struct request_queue *q)
 {
 	if (likely(!blk_queue_dying(q))) {
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fca9b158f4a0..5d0fc165a036 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -860,22 +860,32 @@ static void blk_exit_queue(struct request_queue *q)
 	bdi_put(q->backing_dev_info);
 }
 
-
 /**
- * __blk_release_queue - release a request queue
- * @work: pointer to the release_work member of the request queue to be released
+ * blk_release_queue - releases all allocated resources of the request_queue
+ * @kobj: pointer to a kobject, whose container is a request_queue
+ *
+ * This function releases all allocated resources of the request queue.
+ *
+ * The struct request_queue refcount is incremented with blk_get_queue() and
+ * decremented with blk_put_queue(). Once the refcount reaches 0 this function
+ * is called.
+ *
+ * For drivers that have a request_queue on a gendisk and added with
+ * __device_add_disk() the refcount to request_queue will reach 0 with
+ * the last put_disk() called by the driver. For drivers which don't use
+ * __device_add_disk() this happens with blk_cleanup_queue().
  *
- * Description:
- *     This function is called when a block device is being unregistered. The
- *     process of releasing a request queue starts with blk_cleanup_queue, which
- *     set the appropriate flags and then calls blk_put_queue, that decrements
- *     the reference counter of the request queue. Once the reference counter
- *     of the request queue reaches zero, blk_release_queue is called to release
- *     all allocated resources of the request queue.
+ * Drivers exist which depend on the release of the request_queue to be
+ * synchronous, it should not be deferred.
+ *
+ * Context: can sleep
  */
-static void __blk_release_queue(struct work_struct *work)
+static void blk_release_queue(struct kobject *kobj)
 {
-	struct request_queue *q = container_of(work, typeof(*q), release_work);
+	struct request_queue *q =
+		container_of(kobj, struct request_queue, kobj);
+
+	might_sleep();
 
 	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
 		blk_stat_remove_callback(q, q->poll_cb);
@@ -904,15 +914,6 @@ static void __blk_release_queue(struct work_struct *work)
 	call_rcu(&q->rcu_head, blk_free_queue_rcu);
 }
 
-static void blk_release_queue(struct kobject *kobj)
-{
-	struct request_queue *q =
-		container_of(kobj, struct request_queue, kobj);
-
-	INIT_WORK(&q->release_work, __blk_release_queue);
-	schedule_work(&q->release_work);
-}
-
 static const struct sysfs_ops queue_sysfs_ops = {
 	.show	= queue_attr_show,
 	.store	= queue_attr_store,
diff --git a/block/genhd.c b/block/genhd.c
index c05d509877fa..901567d70390 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -897,11 +897,32 @@ static void invalidate_partition(struct gendisk *disk, int partno)
 	bdput(bdev);
 }
 
+/**
+ * del_gendisk - remove the gendisk
+ * @disk: the struct gendisk to remove
+ *
+ * Removes the gendisk and all its associated resources. This deletes the
+ * partitions associated with the gendisk, and unregisters the associated
+ * request_queue.
+ *
+ * This is the counter to the respective __device_add_disk() call.
+ *
+ * The final removal of the struct gendisk happens when its refcount reaches 0
+ * with put_disk(), which should be called after del_gendisk(), if
+ * __device_add_disk() was used.
+ *
+ * Drivers exist which depend on the release of the gendisk to be synchronous,
+ * it should not be deferred.
+ *
+ * Context: can sleep
+ */
 void del_gendisk(struct gendisk *disk)
 {
 	struct disk_part_iter piter;
 	struct hd_struct *part;
 
+	might_sleep();
+
 	blk_integrity_del(disk);
 	disk_del_events(disk);
 
@@ -992,11 +1013,15 @@ static ssize_t disk_badblocks_store(struct device *dev,
  *
  * This function gets the structure containing partitioning
  * information for the given device @devt.
+ *
+ * Context: can sleep
  */
 struct gendisk *get_gendisk(dev_t devt, int *partno)
 {
 	struct gendisk *disk = NULL;
 
+	might_sleep();
+
 	if (MAJOR(devt) != BLOCK_EXT_MAJOR) {
 		struct kobject *kobj;
 
@@ -1528,10 +1553,31 @@ int disk_expand_part_tbl(struct gendisk *disk, int partno)
 	return 0;
 }
 
+/**
+ * disk_release - releases all allocated resources of the gendisk
+ * @dev: the device representing this disk
+ *
+ * This function releases all allocated resources of the gendisk.
+ *
+ * The struct gendisk refcount is incremented with get_gendisk() or
+ * get_disk_and_module(), and its refcount is decremented with
+ * put_disk_and_module() or put_disk(). Once the refcount reaches 0 this
+ * function is called.
+ *
+ * Drivers which used __device_add_disk() have a gendisk with a request_queue
+ * assigned. Since the request_queue sits on top of the gendisk for these
+ * drivers we also call blk_put_queue() for them, and we expect the
+ * request_queue refcount to reach 0 at this point, and so the request_queue
+ * will also be freed prior to the disk.
+ *
+ * Context: can sleep
+ */
 static void disk_release(struct device *dev)
 {
 	struct gendisk *disk = dev_to_disk(dev);
 
+	might_sleep();
+
 	blk_free_devt(dev->devt);
 	disk_release_events(disk);
 	kfree(disk->random);
@@ -1737,6 +1783,15 @@ struct gendisk *__alloc_disk_node(int minors, int node_id)
 }
 EXPORT_SYMBOL(__alloc_disk_node);
 
+/**
+ * get_disk_and_module - increments the gendisk and gendisk fops module refcount
+ * @disk: the struct gendisk to to increment the refcount for
+ *
+ * This increments the refcount for the struct gendisk, and the gendisk's
+ * fops module owner.
+ *
+ * Context: Any context.
+ */
 struct kobject *get_disk_and_module(struct gendisk *disk)
 {
 	struct module *owner;
@@ -1757,6 +1812,16 @@ struct kobject *get_disk_and_module(struct gendisk *disk)
 }
 EXPORT_SYMBOL(get_disk_and_module);
 
+/**
+ * put_disk - decrements the gendisk refcount
+ * @disk: the struct gendisk to to decrement the refcount for
+ *
+ * This decrements the refcount for the struct gendisk. When this reaches 0
+ * we'll have disk_release() called.
+ *
+ * Context: Any context, but the last reference must not be dropped from
+ *          atomic context.
+ */
 void put_disk(struct gendisk *disk)
 {
 	if (disk)
@@ -1764,9 +1829,15 @@ void put_disk(struct gendisk *disk)
 }
 EXPORT_SYMBOL(put_disk);
 
-/*
+/**
+ * put_disk_and_module - decrements the module and gendisk refcount
+ * @disk: the struct gendisk to to decrement the refcount for
+ *
  * This is a counterpart of get_disk_and_module() and thus also of
  * get_gendisk().
+ *
+ * Context: Any context, but the last reference must not be dropped from
+ *          atomic context.
  */
 void put_disk_and_module(struct gendisk *disk)
 {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index f00bd4042295..3122a93c7277 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -571,8 +571,6 @@ struct request_queue {
 
 	size_t			cmd_size;
 
-	struct work_struct	release_work;
-
 #define BLK_MAX_WRITE_HINTS	5
 	u64			write_hints[BLK_MAX_WRITE_HINTS];
 };
-- 
2.25.1

[PATCH v4 2/5] block: move main block debugfs initialization to its own file

[Luis Chamberlain]

make_request-based drivers and and request-based drivers share some
debugfs code. By moving this into its own file it makes it easier
to expand and audit this shared code.

This patch contains no functional changes.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: yu kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 block/Makefile      |  1 +
 block/blk-core.c    |  9 +--------
 block/blk-debugfs.c | 15 +++++++++++++++
 block/blk.h         |  7 +++++++
 4 files changed, 24 insertions(+), 8 deletions(-)
 create mode 100644 block/blk-debugfs.c

diff --git a/block/Makefile b/block/Makefile
index 206b96e9387f..1d3ab20505d8 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -10,6 +10,7 @@ obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-sysfs.o \
 			blk-mq-sysfs.o blk-mq-cpumap.o blk-mq-sched.o ioctl.o \
 			genhd.o ioprio.o badblocks.o partitions/ blk-rq-qos.o
 
+obj-$(CONFIG_DEBUG_FS)		+= blk-debugfs.o
 obj-$(CONFIG_BOUNCE)		+= bounce.o
 obj-$(CONFIG_BLK_SCSI_REQUEST)	+= scsi_ioctl.o
 obj-$(CONFIG_BLK_DEV_BSG)	+= bsg.o
diff --git a/block/blk-core.c b/block/blk-core.c
index da120fd257fa..0a34b299275e 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -49,10 +49,6 @@
 #include "blk-pm.h"
 #include "blk-rq-qos.h"
 
-#ifdef CONFIG_DEBUG_FS
-struct dentry *blk_debugfs_root;
-#endif
-
 EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_remap);
 EXPORT_TRACEPOINT_SYMBOL_GPL(block_rq_remap);
 EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_complete);
@@ -1813,10 +1809,7 @@ int __init blk_dev_init(void)
 
 	blk_requestq_cachep = kmem_cache_create("request_queue",
 			sizeof(struct request_queue), 0, SLAB_PANIC, NULL);
-
-#ifdef CONFIG_DEBUG_FS
-	blk_debugfs_root = debugfs_create_dir("block", NULL);
-#endif
+	blk_debugfs_register();
 
 	return 0;
 }
diff --git a/block/blk-debugfs.c b/block/blk-debugfs.c
new file mode 100644
index 000000000000..19091e1effc0
--- /dev/null
+++ b/block/blk-debugfs.c
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Shared request-based / make_request-based functionality
+ */
+#include <linux/kernel.h>
+#include <linux/blkdev.h>
+#include <linux/debugfs.h>
+
+struct dentry *blk_debugfs_root;
+
+void blk_debugfs_register(void)
+{
+	blk_debugfs_root = debugfs_create_dir("block", NULL);
+}
diff --git a/block/blk.h b/block/blk.h
index 73bd3b1c6938..ec16e8a6049e 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -456,5 +456,12 @@ struct request_queue *__blk_alloc_queue(int node_id);
 int __bio_add_pc_page(struct request_queue *q, struct bio *bio,
 		struct page *page, unsigned int len, unsigned int offset,
 		bool *same_page);
+#ifdef CONFIG_DEBUG_FS
+void blk_debugfs_register(void);
+#else
+static inline void blk_debugfs_register(void)
+{
+}
+#endif /* CONFIG_DEBUG_FS */
 
 #endif /* BLK_INTERNAL_H */
-- 
2.25.1

[PATCH v4 3/5] blktrace: fix debugfs use after free

[Luis Chamberlain]

On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
merged on v4.12 Omar fixed the original blktrace code for request-based
drivers (multiqueue). This however left in place a possible crash, if you
happen to abuse blktrace while racing to remove / add a device.

We used to use asynchronous removal of the request_queue, and with that
the issue was easier to reproduce. Now that we have reverted to
synchronous removal of the request_queue, the issue is still possible to
reproduce, its however just a bit more difficult.

We essentially run two instances of break-blktrace which add/remove
a loop device, and setup a blktrace and just never tear the blktrace
down. We do this twice in parallel. This is easily reproduced with the
break-blktrace run_0004.sh script.

We can end up with two types of panics each reflecting where we
race, one a failed blktrace setup:

[  252.426751] debugfs: Directory 'loop0' with parent 'block' already present!
[  252.432265] BUG: kernel NULL pointer dereference, address: 00000000000000a0
[  252.436592] #PF: supervisor write access in kernel mode
[  252.439822] #PF: error_code(0x0002) - not-present page
[  252.442967] PGD 0 P4D 0
[  252.444656] Oops: 0002 [#1] SMP NOPTI
[  252.446972] CPU: 10 PID: 1153 Comm: break-blktrace Tainted: G            E     5.7.0-rc2-next-20200420+ #164
[  252.452673] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
[  252.456343] RIP: 0010:down_write+0x15/0x40
[  252.458146] Code: eb ca e8 ae 22 8d ff cc cc cc cc cc cc cc cc cc cc cc cc
               cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00
               00 00 <f0> 48 0f b1 55 00 75 0f 48 8b 04 25 c0 8b 01 00 48 89
               45 08 5d
[  252.463638] RSP: 0018:ffffa626415abcc8 EFLAGS: 00010246
[  252.464950] RAX: 0000000000000000 RBX: ffff958c25f0f5c0 RCX: ffffff8100000000
[  252.466727] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
[  252.468482] RBP: 00000000000000a0 R08: 0000000000000000 R09: 0000000000000001
[  252.470014] R10: 0000000000000000 R11: ffff958d1f9227ff R12: 0000000000000000
[  252.471473] R13: ffff958c25ea5380 R14: ffffffff8cce15f1 R15: 00000000000000a0
[  252.473346] FS:  00007f2e69dee540(0000) GS:ffff958c2fc80000(0000) knlGS:0000000000000000
[  252.475225] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  252.476267] CR2: 00000000000000a0 CR3: 0000000427d10004 CR4: 0000000000360ee0
[  252.477526] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  252.478776] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  252.479866] Call Trace:
[  252.480322]  simple_recursive_removal+0x4e/0x2e0
[  252.481078]  ? debugfs_remove+0x60/0x60
[  252.481725]  ? relay_destroy_buf+0x77/0xb0
[  252.482662]  debugfs_remove+0x40/0x60
[  252.483518]  blk_remove_buf_file_callback+0x5/0x10
[  252.484328]  relay_close_buf+0x2e/0x60
[  252.484930]  relay_open+0x1ce/0x2c0
[  252.485520]  do_blk_trace_setup+0x14f/0x2b0
[  252.486187]  __blk_trace_setup+0x54/0xb0
[  252.486803]  blk_trace_ioctl+0x90/0x140
[  252.487423]  ? do_sys_openat2+0x1ab/0x2d0
[  252.488053]  blkdev_ioctl+0x4d/0x260
[  252.488636]  block_ioctl+0x39/0x40
[  252.489139]  ksys_ioctl+0x87/0xc0
[  252.489675]  __x64_sys_ioctl+0x16/0x20
[  252.490380]  do_syscall_64+0x52/0x180
[  252.491032]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

And the other on the device removal:

[  128.528940] debugfs: Directory 'loop0' with parent 'block' already present!
[  128.615325] BUG: kernel NULL pointer dereference, address: 00000000000000a0
[  128.619537] #PF: supervisor write access in kernel mode
[  128.622700] #PF: error_code(0x0002) - not-present page
[  128.625842] PGD 0 P4D 0
[  128.627585] Oops: 0002 [#1] SMP NOPTI
[  128.629871] CPU: 12 PID: 544 Comm: break-blktrace Tainted: G            E     5.7.0-rc2-next-20200420+ #164
[  128.635595] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
[  128.640471] RIP: 0010:down_write+0x15/0x40
[  128.643041] Code: eb ca e8 ae 22 8d ff cc cc cc cc cc cc cc cc cc cc cc cc
               cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00
               00 00 <f0> 48 0f b1 55 00 75 0f 65 48 8b 04 25 c0 8b 01 00 48 89
               45 08 5d
[  128.650180] RSP: 0018:ffffa9c3c05ebd78 EFLAGS: 00010246
[  128.651820] RAX: 0000000000000000 RBX: ffff8ae9a6370240 RCX: ffffff8100000000
[  128.653942] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
[  128.655720] RBP: 00000000000000a0 R08: 0000000000000002 R09: ffff8ae9afd2d3d0
[  128.657400] R10: 0000000000000056 R11: 0000000000000000 R12: 0000000000000000
[  128.659099] R13: 0000000000000000 R14: 0000000000000003 R15: 00000000000000a0
[  128.660500] FS:  00007febfd995540(0000) GS:ffff8ae9afd00000(0000) knlGS:0000000000000000
[  128.662204] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  128.663426] CR2: 00000000000000a0 CR3: 0000000420042003 CR4: 0000000000360ee0
[  128.664776] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  128.666022] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  128.667282] Call Trace:
[  128.667801]  simple_recursive_removal+0x4e/0x2e0
[  128.668663]  ? debugfs_remove+0x60/0x60
[  128.669368]  debugfs_remove+0x40/0x60
[  128.669985]  blk_trace_free+0xd/0x50
[  128.670593]  __blk_trace_remove+0x27/0x40
[  128.671274]  blk_trace_shutdown+0x30/0x40
[  128.671935]  blk_release_queue+0x95/0xf0
[  128.672589]  kobject_put+0xa5/0x1b0
[  128.673188]  disk_release+0xa2/0xc0
[  128.673786]  device_release+0x28/0x80
[  128.674376]  kobject_put+0xa5/0x1b0
[  128.674915]  loop_remove+0x39/0x50 [loop]
[  128.675511]  loop_control_ioctl+0x113/0x130 [loop]
[  128.676199]  ksys_ioctl+0x87/0xc0
[  128.676708]  __x64_sys_ioctl+0x16/0x20
[  128.677274]  do_syscall_64+0x52/0x180
[  128.677823]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The common theme here is:

debugfs: Directory 'loop0' with parent 'block' already present

This crash happens because of how blktrace uses the debugfs directory
where it places its files. Upon init we always create the same directory
which would be needed by blktrace but we only do this for make_request
drivers (multiqueue) block drivers, but never for request-based block
drivers. Furthermore, that directory is only created on init for the
entire disk. This means that if you use blktrace on a partition, we'll
always be creating a new directory regardless of whether or not you
are doing blktrace on a make_request driver (multiqueue) or a
request-based block drivers.

These directory creations are only associated with a path, and so
when a debugfs_remove() is called it removes everything in its way.
A device removal will remove all blktrace files, and so if a blktrace
is still present a cleanup of blktrace files later will end up trying
to remove dentries pointing to NULL.

We can fix the UAF by using a debugfs directory which moving forward
will always be accessible if debugfs is enabled for both make_request
drivers (multiqueue) and request-based block drivers, *and* for all
partitions upon creation. This ensures that removal of the directories
only happens on device removal and removes the race of the files
underneath an active blktrace.

For partitions we simply symlink to the whole disk's debugfs_dir, as the
debugfs_dir is shared anyway and this limits us to only run one blktrace
for the entire disk.

We special-case a solution for scsi-generic which got blktrace support
added by Christof via commit 6da127ad0918 ("blktrace: Add blktrace
ioctls to SCSI generic devices") so upstream since v2.6.25. scsi-generic
drives use a character device, however behind the scenes we have a scsi
device with a request_queue. How this is used varies by class of driver
(TYPE_DISK, TYPE_TYPE, etc). Care has to be taken into consideration of
the fact that scsi drivers will probe asynchronously but the scsi-generic
class_interface sg_add_device() will complete before. This means
sd_probe() will use device_add_disk() for TYPE_DISK and have its
debugfs_dir created *after* the scsi-generic device is created.

For scsi-generic then we symlink to the real debugfs_dir only during a
blktrace ioctl, but we do this only once. We also have to special-case
yet another solution for drivers which use the bsg queue.

This goes tested with:

  o nvme partitions
  o ISCSI with tgt, and blktracing against scsi-generic with:
    o block
    o tape
    o cdrom
    o media changer

Screenshots of what the debugfs for block looks like after running
blktrace on a system with sg0  which has a raid controllerand then sg1
as the media changer:

 # ls -l /sys/kernel/debug/block
total 0
drwxr-xr-x  3 root root 0 May  9 02:31 bsg
drwxr-xr-x 19 root root 0 May  9 02:31 nvme0n1
drwxr-xr-x 19 root root 0 May  9 02:31 nvme1n1
lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p1 -> nvme1n1
lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p2 -> nvme1n1
lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p3 -> nvme1n1
lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p5 -> nvme1n1
lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p6 -> nvme1n1
drwxr-xr-x  2 root root 0 May  9 02:33 sch0
lrwxrwxrwx  1 root root 0 May  9 02:33 sg0 -> bsg/2:0:0:0
lrwxrwxrwx  1 root root 0 May  9 02:33 sg1 -> sch0
drwxr-xr-x  5 root root 0 May  9 02:31 vda
lrwxrwxrwx  1 root root 0 May  9 02:31 vda1 -> vda

Code for handling the  ebugfs_dir did get more complicatd for
scsi-generic but this is technical debt. For the other types of devices,
this simplifies the code considerably, with the only penalty now being
that we're always creating the request queue debugfs directory for the
request-based block device drivers.

The symlink use also makes it clearer when the request_queue is shared.

This patch is part of the work which disputes the severity of
CVE-2019-19770 which shows this issue is not a core debugfs issue, but
a misuse of debugfs within blktace.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: yu kuai <yukuai3@huawei.com>
Cc: Christof Schmitt <christof.schmitt@de.ibm.com>
Reported-by: syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
Fixes: 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 block/blk-debugfs.c          | 187 +++++++++++++++++++++++++++++++++++
 block/blk-mq-debugfs.c       |   5 -
 block/blk-sysfs.c            |   3 +
 block/blk.h                  |  16 +++
 block/bsg.c                  |   2 +
 block/partitions/core.c      |   9 ++
 drivers/scsi/ch.c            |   1 +
 drivers/scsi/sg.c            |  75 ++++++++++++++
 drivers/scsi/st.c            |   2 +
 include/linux/blkdev.h       |   4 +-
 include/linux/blktrace_api.h |   1 -
 include/linux/genhd.h        |  69 +++++++++++++
 kernel/trace/blktrace.c      |  24 +++--
 13 files changed, 385 insertions(+), 13 deletions(-)

diff --git a/block/blk-debugfs.c b/block/blk-debugfs.c
index 19091e1effc0..d40f12aecf8a 100644
--- a/block/blk-debugfs.c
+++ b/block/blk-debugfs.c
@@ -8,8 +8,195 @@
 #include <linux/debugfs.h>
 
 struct dentry *blk_debugfs_root;
+struct dentry *blk_debugfs_bsg = NULL;
+
+/**
+ * enum blk_debugfs_dir_type - block device debugfs directory type
+ * @BLK_DBG_DIR_BASE: the block device debugfs_dir exists on the base
+ * 	system <system-debugfs-dir>/block/ debugfs directory.
+ * @BLK_DBG_DIR_BSG: the block device debugfs_dir is under the directory
+ * 	<system-debugfs-dir>/block/bsg/
+ */
+enum blk_debugfs_dir_type {
+	BLK_DBG_DIR_BASE = 1,
+	BLK_DBG_DIR_BSG,
+};
 
 void blk_debugfs_register(void)
 {
 	blk_debugfs_root = debugfs_create_dir("block", NULL);
 }
+
+static struct dentry *queue_get_base_dir(enum blk_debugfs_dir_type type)
+{
+	switch (type) {
+	case BLK_DBG_DIR_BASE:
+		return blk_debugfs_root;
+	case BLK_DBG_DIR_BSG:
+		return blk_debugfs_bsg;
+	}
+	return NULL;
+}
+
+static void queue_debugfs_register_type(struct request_queue *q,
+					const char *name,
+					enum blk_debugfs_dir_type type)
+{
+	struct dentry *base_dir = queue_get_base_dir(type);
+
+	q->debugfs_dir = debugfs_create_dir(name, base_dir);
+}
+
+/**
+ * blk_queue_debugfs_register - register the debugfs_dir for the block device
+ * @q: the associated request_queue of the block device
+ * @name: the name of the block device exposed
+ *
+ * This is used to create the debugfs_dir used by the block layer and blktrace.
+ * Drivers which use any of the *add_disk*() calls or variants have this called
+ * automatically for them. This directory is removed automatically on
+ * blk_release_queue() once the request_queue reference count reaches 0.
+ */
+void blk_queue_debugfs_register(struct request_queue *q, const char *name)
+{
+	queue_debugfs_register_type(q, name, BLK_DBG_DIR_BASE);
+}
+EXPORT_SYMBOL_GPL(blk_queue_debugfs_register);
+
+/**
+ * blk_queue_debugfs_unregister - remove the debugfs_dir for the block device
+ * @q: the associated request_queue of the block device
+ *
+ * Removes the debugfs_dir for the request_queue on the associated block device.
+ * This is handled for you on blk_release_queue(), and that should only be
+ * called once.
+ *
+ * Since we don't care where the debugfs_dir was created this is used for all
+ * types of of enum blk_debugfs_dir_type.
+ */
+void blk_queue_debugfs_unregister(struct request_queue *q)
+{
+	debugfs_remove_recursive(q->debugfs_dir);
+}
+
+static struct dentry *queue_debugfs_symlink_type(struct request_queue *q,
+						 const char *src,
+						 const char *dst,
+						 enum blk_debugfs_dir_type type)
+{
+	struct dentry *dentry = ERR_PTR(-EINVAL);
+	char *dir_dst;
+
+	dir_dst = kzalloc(PATH_MAX, GFP_KERNEL);
+	if (!dir_dst)
+		return dentry;
+
+	switch (type) {
+	case BLK_DBG_DIR_BASE:
+		if (dst)
+			snprintf(dir_dst, PATH_MAX, "%s", dst);
+		else if (!IS_ERR_OR_NULL(q->debugfs_dir))
+			snprintf(dir_dst, PATH_MAX, "%s",
+				 q->debugfs_dir->d_name.name);
+		else
+			goto out;
+		break;
+	case BLK_DBG_DIR_BSG:
+		if (dst)
+			snprintf(dir_dst, PATH_MAX, "bsg/%s", dst);
+		else
+			goto out;
+		break;
+	}
+
+	/*
+	 * The base block debugfs directory is always used for the symlinks,
+	 * their target is what changes.
+	 */
+	dentry = debugfs_create_symlink(src, blk_debugfs_root, dir_dst);
+out:
+	kfree(dir_dst);
+
+	return dentry;
+}
+
+/**
+ * blk_queue_debugfs_symlink - symlink to the real block device debugfs_dir
+ * @q: the request queue where we know the debugfs_dir exists or will exist
+ *     eventually. Cannot be NULL.
+ * @src: name of the exposed device we wish to associate to the block device
+ * @dst: the name of the directory to which we want to symlink to, may be NULL
+ *	 if you do not know what this may be, but only if your base block device
+ *	 is not bsg. If you set this to NULL, we will have no other option but
+ *	 to look at the request_queue to infer the name, but you must ensure
+ *	 it is already be set, be mindful of asynchronous probes.
+ *
+ * Some devices don't have a request_queue of their own, however, they have an
+ * association to one and have historically supported using the same
+ * debugfs_dir which has been used to represent the whole disk for blktrace
+ * functionality. Such is the case for partitions and for scsi-generic devices.
+ * They share the same request_queue and debugfs_dir as with the whole disk for
+ * blktrace purposes.  This helper allows such association to be made explicit
+ * and enable blktrace functionality for them. scsi-generic devices representing
+ * scsi device such as block, cdrom, tape, media changer register their own
+ * debug_dir already and share the same request_queue as with scsi-generic, as
+ * such the respective scsi-generic debugfs_dir is just a symlink to these
+ * driver's debugfs_dir.
+ *
+ * To remove use debugfs_remove() on the symlink dentry returned by this
+ * function. The block layer will not clean this up for you, you must remove
+ * it yourself in case of device removal.
+ */
+struct dentry *blk_queue_debugfs_symlink(struct request_queue *q,
+					 const char *src,
+					 const char *dst)
+{
+	return queue_debugfs_symlink_type(q, src, dst, BLK_DBG_DIR_BASE);
+}
+EXPORT_SYMBOL_GPL(blk_queue_debugfs_symlink);
+
+#ifdef CONFIG_BLK_DEV_BSG
+
+void blk_debugfs_register_bsg(void)
+{
+	blk_debugfs_bsg = debugfs_create_dir("bsg", blk_debugfs_root);
+}
+
+/**
+ * blk_queue_debugfs_register_bsg - create the debugfs_dir for bsg block devices
+ * @q: the associated request_queue of the block device
+ * @name: the name of the block device exposed
+ *
+ * This is used to create the debugfs_dir used by the Block layer SCSI generic
+ * (bsg) driver. This is to be used only by the scsi-generic driver on behalf
+ * of scsi devices which work as scsi controllers or transports.
+ *
+ * This directory is cleaned up for all drivers automatically on
+ * blk_release_queue() once the request_queue reference count reaches 0.
+ */
+void blk_queue_debugfs_register_bsg(struct request_queue *q, const char *name)
+{
+	queue_debugfs_register_type(q, name, BLK_DBG_DIR_BSG);
+}
+EXPORT_SYMBOL_GPL(blk_queue_debugfs_register_bsg);
+
+/**
+ * blk_queue_debugfs_symlink_bsg - symlink to the bsg debugfs_dir
+ * @q: the request queue where we know the debugfs_dir exists or will exist
+ *     eventually. Cannot be NULL.
+ * @src: name of the scsi-generic device we wish to associate to the bsg
+ * 	request_queue.
+ * @dst: the name of the bsg request_queue debugfs_dir to which we want to
+ *	 symlink to. This cannot be NULL.
+ *
+ * This is used by scsi-generic devices representing raid controllers /
+ * transport drivers.
+ */
+struct dentry *blk_queue_debugfs_bsg_symlink(struct request_queue *q,
+					     const char *src,
+					     const char *dst)
+{
+	return queue_debugfs_symlink_type(q, src, dst, BLK_DBG_DIR_BSG);
+}
+EXPORT_SYMBOL_GPL(blk_queue_debugfs_bsg_symlink);
+#endif /* CONFIG_BLK_DEV_BSG */
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 96b7a35c898a..08edc3a54114 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -822,9 +822,6 @@ void blk_mq_debugfs_register(struct request_queue *q)
 	struct blk_mq_hw_ctx *hctx;
 	int i;
 
-	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
-					    blk_debugfs_root);
-
 	debugfs_create_files(q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
 
 	/*
@@ -855,9 +852,7 @@ void blk_mq_debugfs_register(struct request_queue *q)
 
 void blk_mq_debugfs_unregister(struct request_queue *q)
 {
-	debugfs_remove_recursive(q->debugfs_dir);
 	q->sched_debugfs_dir = NULL;
-	q->debugfs_dir = NULL;
 }
 
 static void blk_mq_debugfs_register_ctx(struct blk_mq_hw_ctx *hctx,
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 5d0fc165a036..1d151f19bd87 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -905,6 +905,7 @@ static void blk_release_queue(struct kobject *kobj)
 
 	blk_trace_shutdown(q);
 
+	blk_queue_debugfs_unregister(q);
 	if (queue_is_mq(q))
 		blk_mq_debugfs_unregister(q);
 
@@ -976,6 +977,8 @@ int blk_register_queue(struct gendisk *disk)
 		goto unlock;
 	}
 
+	blk_queue_debugfs_register(q, kobject_name(q->kobj.parent));
+
 	if (queue_is_mq(q)) {
 		__blk_mq_register_dev(dev, q);
 		blk_mq_debugfs_register(q);
diff --git a/block/blk.h b/block/blk.h
index ec16e8a6049e..f7ace11c8bd1 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -458,10 +458,26 @@ int __bio_add_pc_page(struct request_queue *q, struct bio *bio,
 		bool *same_page);
 #ifdef CONFIG_DEBUG_FS
 void blk_debugfs_register(void);
+void blk_queue_debugfs_unregister(struct request_queue *q);
+void blk_part_debugfs_register(struct hd_struct *p, const char *name);
+void blk_part_debugfs_unregister(struct hd_struct *p);
 #else
 static inline void blk_debugfs_register(void)
 {
 }
+
+static inline void blk_queue_debugfs_unregister(struct request_queue *q)
+{
+}
+
+static inline void blk_part_debugfs_register(struct hd_struct *p,
+					     const char *name)
+{
+}
+
+static inline void blk_part_debugfs_unregister(struct hd_struct *p)
+{
+}
 #endif /* CONFIG_DEBUG_FS */
 
 #endif /* BLK_INTERNAL_H */
diff --git a/block/bsg.c b/block/bsg.c
index d7bae94b64d9..bfb1036858c4 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -503,6 +503,8 @@ static int __init bsg_init(void)
 	if (ret)
 		goto unregister_chrdev;
 
+	blk_debugfs_register_bsg();
+
 	printk(KERN_INFO BSG_DESCRIPTION " version " BSG_VERSION
 	       " loaded (major %d)\n", bsg_major);
 	return 0;
diff --git a/block/partitions/core.c b/block/partitions/core.c
index 873999e2e2f2..a96b2418e70d 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -10,6 +10,7 @@
 #include <linux/vmalloc.h>
 #include <linux/blktrace_api.h>
 #include <linux/raid/detect.h>
+#include <linux/debugfs.h>
 #include "check.h"
 
 static int (*check_part[])(struct parsed_partitions *) = {
@@ -309,6 +310,9 @@ void delete_partition(struct gendisk *disk, struct hd_struct *part)
 	struct disk_part_tbl *ptbl =
 		rcu_dereference_protected(disk->part_tbl, 1);
 
+#ifdef CONFIG_DEBUG_FS
+	debugfs_remove(part->debugfs_sym);
+#endif
 	rcu_assign_pointer(ptbl->part[part->partno], NULL);
 	rcu_assign_pointer(ptbl->last_lookup, NULL);
 	kobject_put(part->holder_dir);
@@ -450,6 +454,11 @@ static struct hd_struct *add_partition(struct gendisk *disk, int partno,
 	/* everything is up and running, commence */
 	rcu_assign_pointer(ptbl->part[partno], p);
 
+#ifdef CONFIG_DEBUG_FS
+	p->debugfs_sym = blk_queue_debugfs_symlink(disk->queue, dev_name(pdev),
+						   disk->disk_name);
+#endif
+
 	/* suppress uevent if the disk suppresses it */
 	if (!dev_get_uevent_suppress(ddev))
 		kobject_uevent(&pdev->kobj, KOBJ_ADD);
diff --git a/drivers/scsi/ch.c b/drivers/scsi/ch.c
index cb74ab1ae5a4..5dfabc04bfef 100644
--- a/drivers/scsi/ch.c
+++ b/drivers/scsi/ch.c
@@ -971,6 +971,7 @@ static int ch_probe(struct device *dev)
 
 	mutex_unlock(&ch->lock);
 	dev_set_drvdata(dev, ch);
+	blk_queue_debugfs_register(sd->request_queue, dev_name(class_dev));
 	sdev_printk(KERN_INFO, sd, "Attached scsi changer %s\n", ch->name);
 
 	return 0;
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 20472aaaf630..6fa201086e59 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -47,6 +47,7 @@ static int sg_version_num = 30536;	/* 2 digits for each component */
 #include <linux/ratelimit.h>
 #include <linux/uio.h>
 #include <linux/cred.h> /* for sg_check_file_access() */
+#include <linux/debugfs.h>
 
 #include "scsi.h"
 #include <scsi/scsi_dbg.h>
@@ -169,6 +170,10 @@ typedef struct sg_device { /* holds the state of each scsi generic device */
 	struct gendisk *disk;
 	struct cdev * cdev;	/* char_dev [sysfs: /sys/cdev/major/sg<n>] */
 	struct kref d_ref;
+#ifdef CONFIG_DEBUG_FS
+	bool debugfs_set;
+	struct dentry *debugfs_sym;
+#endif
 } Sg_device;
 
 /* tasklet or soft irq callback */
@@ -914,6 +919,72 @@ static int put_compat_request_table(struct compat_sg_req_info __user *o,
 }
 #endif
 
+#ifdef CONFIG_DEBUG_FS
+/*
+ * For scsi-generic devices like TYPE_DISK will re-use the scsi_device
+ * request_queue on their driver for their disk and later device_add_disk() it,
+ * we want its respective scsi-generic debugfs_dir to just be a symlink to the
+ * one created on the real scsi device probe.
+ *
+ * We use this on the ioctl path instead of sg_add_device() since some driver
+ * probes can run asynchronously. Such is the case for scsi devices of
+ * TYPE_DISK, and the class interface currently has no callbacks once a device
+ * driver probe has completed its probe. We don't use wait_for_device_probe()
+ * on sg_add_device() as that would defeat the purpose of using asynchronous
+ * probe.
+ */
+static void sg_init_blktrace_setup(Sg_device *sdp)
+{
+	struct scsi_device *scsidp = sdp->device;
+	struct device *scsi_dev = &scsidp->sdev_gendev;
+	struct gendisk *sg_disk = sdp->disk;
+	struct request_queue *q = scsidp->request_queue;
+
+	/*
+	 * Although debugfs is used for debugging purposes and we
+	 * typically don't care about the return value, we do here
+	 * because we use it for userspace to ensure blktrace works.
+	 *
+	 * Instead of always just checking for the return value though,
+	 * just try setting this once, if the first time failed we don't
+	 * try again.
+	 */
+	if (sdp->debugfs_set)
+		return;
+
+	switch (sdp->device->type) {
+	case TYPE_RAID:
+		/*
+		 * We do the registration for bsg here to keep bsg scsi_device
+		 * opaque. If bsg is disabled we just create the debugfs_dir on
+		 * the base block debugfs_dir and scsi-generic symlinks to it.
+		 */
+		blk_queue_debugfs_register_bsg(q, dev_name(scsi_dev));
+		sdp->debugfs_sym =
+			blk_queue_debugfs_bsg_symlink(q,
+						      sg_disk->disk_name,
+						      dev_name(scsi_dev));
+		break;
+	default:
+		/*
+		 * We don't know scsi_device probed device name (this is
+		 * different from the scsi_device name). This is opaque to
+		 * scsi-generic, so we use the request_queue to infer the name
+		 * based on the set debugfs_dir.
+		 */
+		sdp->debugfs_sym = blk_queue_debugfs_symlink(q,
+							     sg_disk->disk_name,
+							     NULL);
+		break;
+	}
+	sdp->debugfs_set = true;
+}
+#else
+static void sg_init_blktrace_setup(Sg_device *sdp)
+{
+}
+#endif
+
 static long
 sg_ioctl_common(struct file *filp, Sg_device *sdp, Sg_fd *sfp,
 		unsigned int cmd_in, void __user *p)
@@ -1117,6 +1188,7 @@ sg_ioctl_common(struct file *filp, Sg_device *sdp, Sg_fd *sfp,
 		return put_user(max_sectors_bytes(sdp->device->request_queue),
 				ip);
 	case BLKTRACESETUP:
+		sg_init_blktrace_setup(sdp);
 		return blk_trace_setup(sdp->device->request_queue,
 				       sdp->disk->disk_name,
 				       MKDEV(SCSI_GENERIC_MAJOR, sdp->index),
@@ -1644,6 +1716,9 @@ sg_remove_device(struct device *cl_dev, struct class_interface *cl_intf)
 
 	sysfs_remove_link(&scsidp->sdev_gendev.kobj, "generic");
 	device_destroy(sg_sysfs_class, MKDEV(SCSI_GENERIC_MAJOR, sdp->index));
+#ifdef CONFIG_DEBUG_FS
+	debugfs_remove(sdp->debugfs_sym);
+#endif
 	cdev_del(sdp->cdev);
 	sdp->cdev = NULL;
 
diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index 4bf4ab3b70f4..fb3c0546803a 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -4417,6 +4417,8 @@ static int st_probe(struct device *dev)
 	if (error)
 		goto out_remove_devs;
 	scsi_autopm_put_device(SDp);
+	blk_queue_debugfs_register(tpnt->device->request_queue,
+				   tape_name(tpnt));
 
 	sdev_printk(KERN_NOTICE, SDp,
 		    "Attached scsi tape %s\n", tape_name(tpnt));
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3122a93c7277..9b12fcc94572 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -561,8 +561,10 @@ struct request_queue {
 	struct list_head	tag_set_list;
 	struct bio_set		bio_split;
 
-#ifdef CONFIG_BLK_DEBUG_FS
+#ifdef CONFIG_DEBUG_FS
 	struct dentry		*debugfs_dir;
+#endif
+#ifdef CONFIG_BLK_DEBUG_FS
 	struct dentry		*sched_debugfs_dir;
 	struct dentry		*rqos_debugfs_dir;
 #endif
diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
index 3b6ff5902edc..eb6db276e293 100644
--- a/include/linux/blktrace_api.h
+++ b/include/linux/blktrace_api.h
@@ -22,7 +22,6 @@ struct blk_trace {
 	u64 end_lba;
 	u32 pid;
 	u32 dev;
-	struct dentry *dir;
 	struct dentry *dropped_file;
 	struct dentry *msg_file;
 	struct list_head running_list;
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index f9c226f9546a..71b7896365b3 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -86,6 +86,9 @@ struct hd_struct {
 #endif
 	struct percpu_ref ref;
 	struct rcu_work rcu_work;
+#ifdef CONFIG_DEBUG_FS
+	struct dentry *debugfs_sym;
+#endif
 };
 
 /**
@@ -391,4 +394,70 @@ static inline dev_t blk_lookup_devt(const char *name, int partno)
 }
 #endif /* CONFIG_BLOCK */
 
+#ifdef CONFIG_DEBUG_FS
+void blk_queue_debugfs_register(struct request_queue *q, const char *name);
+struct dentry *blk_queue_debugfs_symlink(struct request_queue *q,
+					 const char *src,
+					 const char *dst);
+#ifdef CONFIG_BLK_DEV_BSG
+void blk_debugfs_register_bsg(void);
+void blk_queue_debugfs_register_bsg(struct request_queue *q, const char *name);
+struct dentry *blk_queue_debugfs_bsg_symlink(struct request_queue *q,
+					     const char *src,
+					     const char *dst);
+#else
+
+static inline void blk_debugfs_register_bsg(void)
+{
+}
+
+/* If bsg is not enabled we use the base directory */
+static inline void blk_queue_debugfs_register_bsg(struct request_queue *q,
+						  const char *name)
+{
+	blk_queue_debugfs_register(q, name);
+}
+
+static inline
+struct dentry *blk_queue_debugfs_bsg_symlink(struct request_queue *q,
+					     const char *src,
+					     const char *dst)
+{
+	return blk_queue_debugfs_symlink(q, src, dst);
+}
+
+#endif /* CONFIG_BLK_DEV_BSG */
+#else  /* ! CONFIG_DEBUG_FS */
+static inline void blk_queue_debugfs_register(struct request_queue *q,
+					      const char *name)
+{
+}
+
+struct dentry *blk_queue_debugfs_symlink(struct request_queue *q,
+					 const char *src,
+					 const char *dst)
+{
+	return ERR_PTR(-ENODEV);
+}
+
+#ifdef CONFIG_BLK_DEV_BSG
+static inline void blk_debugfs_register_bsg(void)
+{
+}
+#endif /* CONFIG_BLK_DEV_BSG */
+
+static inline void blk_queue_debugfs_register_bsg(struct request_queue *q,
+						  const char *name)
+{
+}
+
+static inline
+struct dentry *blk_queue_debugfs_bsg_symlink(struct request_queue *q,
+					     const char *src,
+					     const char *dst)
+{
+	return ERR_PTR(-ENODEV);
+}
+#endif /* CONFIG_DEBUG_FS */
+
 #endif /* _LINUX_GENHD_H */
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index ca39dc3230cb..6c10a1427de2 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -311,7 +311,6 @@ static void blk_trace_free(struct blk_trace *bt)
 	debugfs_remove(bt->msg_file);
 	debugfs_remove(bt->dropped_file);
 	relay_close(bt->rchan);
-	debugfs_remove(bt->dir);
 	free_percpu(bt->sequence);
 	free_percpu(bt->msg_data);
 	kfree(bt);
@@ -509,9 +508,24 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 
 	ret = -ENOENT;
 
-	dir = debugfs_lookup(buts->name, blk_debugfs_root);
-	if (!dir)
-		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
+	dir = q->debugfs_dir;
+
+	/*
+	 * Although the directory here is from debugfs, and we typically do not
+	 * care about NULL dirs as debugfs is typically only used for debugging,
+	 * we rely on the directory to exist to place files which we then use
+	 * for blktrace userspace functionality. Without this directory
+	 * blktrace would not work. Enabling blktrace functionality enables
+	 * debugfs too, as such, we *really* do want to check for this and must
+	 * ensure it was set before chugging on. If NULL were used below, we'd
+	 * also end up creating the debugfs files under the block root
+	 * directory, which we definitely do not want.
+	 */
+	if (IS_ERR_OR_NULL(dir)) {
+		pr_warn("debugfs_dir not present for %s so skipping\n",
+			buts->name);
+		goto err;
+	}
 
 	bt->dev = dev;
 	atomic_set(&bt->dropped, 0);
@@ -551,8 +565,6 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 
 	ret = 0;
 err:
-	if (dir && !bt->dir)
-		dput(dir);
 	if (ret)
 		blk_trace_free(bt);
 	return ret;
-- 
2.25.1

[PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls

[Luis Chamberlain]

We use one blktrace per request_queue, that means one per the entire
disk.  So we cannot run one blktrace on say /dev/vda and then /dev/vda1,
or just two calls on /dev/vda.

We check for concurrent setup only at the very end of the blktrace setup though.

If we try to run two concurrent blktraces on the same block device the
second one will fail, and the first one seems to go on. However when
one tries to kill the first one one will see things like this:

The kernel will show these:

```
debugfs: File 'dropped' in directory 'nvme1n1' already present!
debugfs: File 'msg' in directory 'nvme1n1' already present!
debugfs: File 'trace0' in directory 'nvme1n1' already present!
``

And userspace just sees this error message for the second call:

```
blktrace /dev/nvme1n1
BLKTRACESETUP(2) /dev/nvme1n1 failed: 5/Input/output error
```

The first userspace process #1 will also claim that the files
were taken underneath their nose as well. The files are taken
away form the first process given that when the second blktrace
fails, it will follow up with a BLKTRACESTOP and BLKTRACETEARDOWN.
This means that even if go-happy process #1 is waiting for blktrace
data, we *have* been asked to take teardown the blktrace.

This can easily be reproduced with break-blktrace [0] run_0005.sh test.

Just break out early if we know we're already going to fail, this will
prevent trying to create the files all over again, which we know still
exist.

[0] https://github.com/mcgrof/break-blktrace
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 kernel/trace/blktrace.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 6c10a1427de2..bd5ec2184d46 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -3,6 +3,9 @@
  * Copyright (C) 2006 Jens Axboe <axboe@kernel.dk>
  *
  */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/kernel.h>
 #include <linux/blkdev.h>
 #include <linux/blktrace_api.h>
@@ -493,6 +496,12 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	 */
 	strreplace(buts->name, '/', '_');
 
+	if (q->blk_trace) {
+		pr_warn("Concurrent blktraces are not allowed on %s\n",
+			buts->name);
+		return -EBUSY;
+	}
+
 	bt = kzalloc(sizeof(*bt), GFP_KERNEL);
 	if (!bt)
 		return -ENOMEM;
-- 
2.25.1

[PATCH v4 5/5] loop: be paranoid on exit and prevent new additions / removals

[Luis Chamberlain]

Be pedantic on removal as well and hold the mutex.
This should prevent uses of addition while we exit.

I cannot trigger an issue with this though, this is
just a fix done through code inspection.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 drivers/block/loop.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 14372df0f354..54fbcbd930de 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -2333,6 +2333,8 @@ static void __exit loop_exit(void)
 
 	range = max_loop ? max_loop << part_shift : 1UL << MINORBITS;
 
+	mutex_lock(&loop_ctl_mutex);
+
 	idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
 	idr_destroy(&loop_index_idr);
 
@@ -2340,6 +2342,8 @@ static void __exit loop_exit(void)
 	unregister_blkdev(LOOP_MAJOR, "loop");
 
 	misc_deregister(&loop_misc);
+
+	mutex_unlock(&loop_ctl_mutex);
 }
 
 module_init(loop_init);
-- 
2.25.1

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1771 bytes --]

Hi Luis,

I love your patch! Yet something to improve:

[auto build test ERROR on block/for-next]
[also build test ERROR on next-20200508]
[cannot apply to mkp-scsi/for-next scsi/for-next linus/master v5.7-rc4]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Luis-Chamberlain/block-fix-blktrace-debugfs-use-after-free/20200509-140139
base:   https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git for-next
config: um-allnoconfig (attached as .config)
compiler: gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=um 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   ld: drivers/char/random.o: in function `blk_queue_debugfs_symlink':
>> random.c:(.text+0x10f0): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:do_mounts.c:(.text+0x10): first defined here
   ld: drivers/base/core.o: in function `blk_queue_debugfs_symlink':
   core.c:(.text+0x800): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:do_mounts.c:(.text+0x10): first defined here
   ld: drivers/base/class.o: in function `blk_queue_debugfs_symlink':
   class.c:(.text+0xd0): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:do_mounts.c:(.text+0x10): first defined here

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org

[-- Attachment #2: config.gz --]
[-- Type: application/gzip, Size: 4609 bytes --]

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1942 bytes --]

Hi Luis,

I love your patch! Yet something to improve:

[auto build test ERROR on block/for-next]
[also build test ERROR on next-20200508]
[cannot apply to mkp-scsi/for-next scsi/for-next linus/master v5.7-rc4]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Luis-Chamberlain/block-fix-blktrace-debugfs-use-after-free/20200509-140139
base:   https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git for-next
config: parisc-allnoconfig (attached as .config)
compiler: hppa-linux-gcc (GCC) 9.3.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day GCC_VERSION=9.3.0 make.cross ARCH=parisc 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   hppa-linux-ld: drivers/char/random.o: in function `blk_queue_debugfs_symlink':
>> (.text+0x1130): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:(.text+0x1c): first defined here
   hppa-linux-ld: drivers/base/core.o: in function `blk_queue_debugfs_symlink':
   (.text+0xc18): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:(.text+0x1c): first defined here
   hppa-linux-ld: drivers/base/class.o: in function `blk_queue_debugfs_symlink':
   (.text+0xf8): multiple definition of `blk_queue_debugfs_symlink'; init/do_mounts.o:(.text+0x1c): first defined here

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org

[-- Attachment #2: config.gz --]
[-- Type: application/gzip, Size: 5579 bytes --]

Re: [PATCH v4 1/5] block: revert back to synchronous request_queue removal

[Bart Van Assche]

On 2020-05-08 20:10, Luis Chamberlain wrote:
> We revert back to synchronous request_queue removal because asynchronous
> removal creates a regression with expected userspace interaction with
> several drivers. An example is when removing the loopback driver, one
> uses ioctls from userspace to do so, but upon return and if successful,
> one expects the device to be removed. Likewise if one races to add another
> device the new one may not be added as it is still being removed. This was
> expected behavior before and it now fails as the device is still present
> and busy still. Moving to asynchronous request_queue removal could have
> broken many scripts which relied on the removal to have been completed if
> there was no error. Document this expectation as well so that this
> doesn't regress userspace again.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[Bart Van Assche]

On 2020-05-08 20:10, Luis Chamberlain wrote:
> Screenshots of what the debugfs for block looks like after running
> blktrace on a system with sg0  which has a raid controllerand then sg1
> as the media changer:
> 
>  # ls -l /sys/kernel/debug/block
> total 0
> drwxr-xr-x  3 root root 0 May  9 02:31 bsg
> drwxr-xr-x 19 root root 0 May  9 02:31 nvme0n1
> drwxr-xr-x 19 root root 0 May  9 02:31 nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p1 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p2 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p3 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p5 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p6 -> nvme1n1
> drwxr-xr-x  2 root root 0 May  9 02:33 sch0
> lrwxrwxrwx  1 root root 0 May  9 02:33 sg0 -> bsg/2:0:0:0
> lrwxrwxrwx  1 root root 0 May  9 02:33 sg1 -> sch0
> drwxr-xr-x  5 root root 0 May  9 02:31 vda
> lrwxrwxrwx  1 root root 0 May  9 02:31 vda1 -> vda

So this patch creates one soft link per partition at partition creation
time instead of letting the blktrace code create one directory per
partition when tracing starts? Does this break running blktrace
simultaneously for a partition and for the entire block device?

> +static struct dentry *queue_debugfs_symlink_type(struct request_queue *q,
> +						 const char *src,
> +						 const char *dst,
> +						 enum blk_debugfs_dir_type type)
> +{
> +	struct dentry *dentry = ERR_PTR(-EINVAL);
> +	char *dir_dst;
> +
> +	dir_dst = kzalloc(PATH_MAX, GFP_KERNEL);
> +	if (!dir_dst)
> +		return dentry;
> +
> +	switch (type) {
> +	case BLK_DBG_DIR_BASE:
> +		if (dst)
> +			snprintf(dir_dst, PATH_MAX, "%s", dst);
> +		else if (!IS_ERR_OR_NULL(q->debugfs_dir))
> +			snprintf(dir_dst, PATH_MAX, "%s",
> +				 q->debugfs_dir->d_name.name);
> +		else
> +			goto out;
> +		break;
> +	case BLK_DBG_DIR_BSG:
> +		if (dst)
> +			snprintf(dir_dst, PATH_MAX, "bsg/%s", dst);
> +		else
> +			goto out;
> +		break;
> +	}
> +
> +	/*
> +	 * The base block debugfs directory is always used for the symlinks,
> +	 * their target is what changes.
> +	 */
> +	dentry = debugfs_create_symlink(src, blk_debugfs_root, dir_dst);
> +out:
> +	kfree(dir_dst);
> +
> +	return dentry;
> +}

Please use kasprintf() instead of k?alloc() followed by snprintf().

Thanks,

Bart.

Re: [PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls

[Bart Van Assche]

On 2020-05-08 20:10, Luis Chamberlain wrote:
> @@ -493,6 +496,12 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
>  	 */
>  	strreplace(buts->name, '/', '_');
>  
> +	if (q->blk_trace) {
> +		pr_warn("Concurrent blktraces are not allowed on %s\n",
> +			buts->name);
> +		return -EBUSY;
> +	}
> +
>  	bt = kzalloc(sizeof(*bt), GFP_KERNEL);
>  	if (!bt)
>  		return -ENOMEM;

Is this really sufficient? Shouldn't concurrent do_blk_trace_setup()
calls that refer to the same request queue be serialized to really
prevent that debugfs attribute creation fails?

How about using the block device name instead of the partition name in
the error message since the concurrency context is the block device and
not the partition?

Thanks,

Bart.

Re: [PATCH v4 1/5] block: revert back to synchronous request_queue removal

[Greg KH]

On Sat, May 09, 2020 at 03:10:54AM +0000, Luis Chamberlain wrote:
> Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
> v4.12 moved the work behind blk_release_queue() into a workqueue after a
> splat floated around which indicated some work on blk_release_queue()
> could sleep in blk_exit_rl(). This splat would be possible when a driver
> called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
> as its final call) from an atomic context.
> 
> blk_put_queue() decrements the refcount for the request_queue kobject,
> and upon reaching 0 blk_release_queue() is called. Although blk_exit_rl()
> is now removed through commit db6d9952356 ("block: remove request_list code")
> on v5.0, we reserve the right to be able to sleep within blk_release_queue()
> context.
> 
> The last reference for the request_queue must not be called from atomic
> context. *When* the last reference to the request_queue reaches 0 varies,
> and so let's take the opportunity to document when that is expected to
> happen and also document the context of the related calls as best as possible
> so we can avoid future issues, and with the hopes that the synchronous
> request_queue removal sticks.
> 
> We revert back to synchronous request_queue removal because asynchronous
> removal creates a regression with expected userspace interaction with
> several drivers. An example is when removing the loopback driver, one
> uses ioctls from userspace to do so, but upon return and if successful,
> one expects the device to be removed. Likewise if one races to add another
> device the new one may not be added as it is still being removed. This was
> expected behavior before and it now fails as the device is still present
> and busy still. Moving to asynchronous request_queue removal could have
> broken many scripts which relied on the removal to have been completed if
> there was no error. Document this expectation as well so that this
> doesn't regress userspace again.
> 
> Using asynchronous request_queue removal however has helped us find
> other bugs. In the future we can test what could break with this
> arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.

You are adding documenation and might_sleep() calls all over the place
in here, making the "real" change in the patch hard to pick out.

How about you split this up into 3 patches, one for documentation, one
for might_sleep() and one for the real change?  Or maybe just 2 patches,
but what you have here seems excessive.

thanks,

greg k-h

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[Greg KH]

On Sat, May 09, 2020 at 03:10:56AM +0000, Luis Chamberlain wrote:
> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> merged on v4.12 Omar fixed the original blktrace code for request-based
> drivers (multiqueue). This however left in place a possible crash, if you
> happen to abuse blktrace while racing to remove / add a device.
> 
> We used to use asynchronous removal of the request_queue, and with that
> the issue was easier to reproduce. Now that we have reverted to
> synchronous removal of the request_queue, the issue is still possible to
> reproduce, its however just a bit more difficult.
> 
> We essentially run two instances of break-blktrace which add/remove
> a loop device, and setup a blktrace and just never tear the blktrace
> down. We do this twice in parallel. This is easily reproduced with the
> break-blktrace run_0004.sh script.
> 
> We can end up with two types of panics each reflecting where we
> race, one a failed blktrace setup:
> 
> [  252.426751] debugfs: Directory 'loop0' with parent 'block' already present!
> [  252.432265] BUG: kernel NULL pointer dereference, address: 00000000000000a0
> [  252.436592] #PF: supervisor write access in kernel mode
> [  252.439822] #PF: error_code(0x0002) - not-present page
> [  252.442967] PGD 0 P4D 0
> [  252.444656] Oops: 0002 [#1] SMP NOPTI
> [  252.446972] CPU: 10 PID: 1153 Comm: break-blktrace Tainted: G            E     5.7.0-rc2-next-20200420+ #164
> [  252.452673] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> [  252.456343] RIP: 0010:down_write+0x15/0x40
> [  252.458146] Code: eb ca e8 ae 22 8d ff cc cc cc cc cc cc cc cc cc cc cc cc
>                cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00
>                00 00 <f0> 48 0f b1 55 00 75 0f 48 8b 04 25 c0 8b 01 00 48 89
>                45 08 5d
> [  252.463638] RSP: 0018:ffffa626415abcc8 EFLAGS: 00010246
> [  252.464950] RAX: 0000000000000000 RBX: ffff958c25f0f5c0 RCX: ffffff8100000000
> [  252.466727] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
> [  252.468482] RBP: 00000000000000a0 R08: 0000000000000000 R09: 0000000000000001
> [  252.470014] R10: 0000000000000000 R11: ffff958d1f9227ff R12: 0000000000000000
> [  252.471473] R13: ffff958c25ea5380 R14: ffffffff8cce15f1 R15: 00000000000000a0
> [  252.473346] FS:  00007f2e69dee540(0000) GS:ffff958c2fc80000(0000) knlGS:0000000000000000
> [  252.475225] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  252.476267] CR2: 00000000000000a0 CR3: 0000000427d10004 CR4: 0000000000360ee0
> [  252.477526] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  252.478776] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [  252.479866] Call Trace:
> [  252.480322]  simple_recursive_removal+0x4e/0x2e0
> [  252.481078]  ? debugfs_remove+0x60/0x60
> [  252.481725]  ? relay_destroy_buf+0x77/0xb0
> [  252.482662]  debugfs_remove+0x40/0x60
> [  252.483518]  blk_remove_buf_file_callback+0x5/0x10
> [  252.484328]  relay_close_buf+0x2e/0x60
> [  252.484930]  relay_open+0x1ce/0x2c0
> [  252.485520]  do_blk_trace_setup+0x14f/0x2b0
> [  252.486187]  __blk_trace_setup+0x54/0xb0
> [  252.486803]  blk_trace_ioctl+0x90/0x140
> [  252.487423]  ? do_sys_openat2+0x1ab/0x2d0
> [  252.488053]  blkdev_ioctl+0x4d/0x260
> [  252.488636]  block_ioctl+0x39/0x40
> [  252.489139]  ksys_ioctl+0x87/0xc0
> [  252.489675]  __x64_sys_ioctl+0x16/0x20
> [  252.490380]  do_syscall_64+0x52/0x180
> [  252.491032]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> And the other on the device removal:
> 
> [  128.528940] debugfs: Directory 'loop0' with parent 'block' already present!
> [  128.615325] BUG: kernel NULL pointer dereference, address: 00000000000000a0
> [  128.619537] #PF: supervisor write access in kernel mode
> [  128.622700] #PF: error_code(0x0002) - not-present page
> [  128.625842] PGD 0 P4D 0
> [  128.627585] Oops: 0002 [#1] SMP NOPTI
> [  128.629871] CPU: 12 PID: 544 Comm: break-blktrace Tainted: G            E     5.7.0-rc2-next-20200420+ #164
> [  128.635595] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> [  128.640471] RIP: 0010:down_write+0x15/0x40
> [  128.643041] Code: eb ca e8 ae 22 8d ff cc cc cc cc cc cc cc cc cc cc cc cc
>                cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00
>                00 00 <f0> 48 0f b1 55 00 75 0f 65 48 8b 04 25 c0 8b 01 00 48 89
>                45 08 5d
> [  128.650180] RSP: 0018:ffffa9c3c05ebd78 EFLAGS: 00010246
> [  128.651820] RAX: 0000000000000000 RBX: ffff8ae9a6370240 RCX: ffffff8100000000
> [  128.653942] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
> [  128.655720] RBP: 00000000000000a0 R08: 0000000000000002 R09: ffff8ae9afd2d3d0
> [  128.657400] R10: 0000000000000056 R11: 0000000000000000 R12: 0000000000000000
> [  128.659099] R13: 0000000000000000 R14: 0000000000000003 R15: 00000000000000a0
> [  128.660500] FS:  00007febfd995540(0000) GS:ffff8ae9afd00000(0000) knlGS:0000000000000000
> [  128.662204] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  128.663426] CR2: 00000000000000a0 CR3: 0000000420042003 CR4: 0000000000360ee0
> [  128.664776] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  128.666022] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [  128.667282] Call Trace:
> [  128.667801]  simple_recursive_removal+0x4e/0x2e0
> [  128.668663]  ? debugfs_remove+0x60/0x60
> [  128.669368]  debugfs_remove+0x40/0x60
> [  128.669985]  blk_trace_free+0xd/0x50
> [  128.670593]  __blk_trace_remove+0x27/0x40
> [  128.671274]  blk_trace_shutdown+0x30/0x40
> [  128.671935]  blk_release_queue+0x95/0xf0
> [  128.672589]  kobject_put+0xa5/0x1b0
> [  128.673188]  disk_release+0xa2/0xc0
> [  128.673786]  device_release+0x28/0x80
> [  128.674376]  kobject_put+0xa5/0x1b0
> [  128.674915]  loop_remove+0x39/0x50 [loop]
> [  128.675511]  loop_control_ioctl+0x113/0x130 [loop]
> [  128.676199]  ksys_ioctl+0x87/0xc0
> [  128.676708]  __x64_sys_ioctl+0x16/0x20
> [  128.677274]  do_syscall_64+0x52/0x180
> [  128.677823]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> The common theme here is:
> 
> debugfs: Directory 'loop0' with parent 'block' already present
> 
> This crash happens because of how blktrace uses the debugfs directory
> where it places its files. Upon init we always create the same directory
> which would be needed by blktrace but we only do this for make_request
> drivers (multiqueue) block drivers, but never for request-based block
> drivers. Furthermore, that directory is only created on init for the
> entire disk. This means that if you use blktrace on a partition, we'll
> always be creating a new directory regardless of whether or not you
> are doing blktrace on a make_request driver (multiqueue) or a
> request-based block drivers.
> 
> These directory creations are only associated with a path, and so
> when a debugfs_remove() is called it removes everything in its way.
> A device removal will remove all blktrace files, and so if a blktrace
> is still present a cleanup of blktrace files later will end up trying
> to remove dentries pointing to NULL.
> 
> We can fix the UAF by using a debugfs directory which moving forward
> will always be accessible if debugfs is enabled for both make_request
> drivers (multiqueue) and request-based block drivers, *and* for all
> partitions upon creation. This ensures that removal of the directories
> only happens on device removal and removes the race of the files
> underneath an active blktrace.
> 
> For partitions we simply symlink to the whole disk's debugfs_dir, as the
> debugfs_dir is shared anyway and this limits us to only run one blktrace
> for the entire disk.
> 
> We special-case a solution for scsi-generic which got blktrace support
> added by Christof via commit 6da127ad0918 ("blktrace: Add blktrace
> ioctls to SCSI generic devices") so upstream since v2.6.25. scsi-generic
> drives use a character device, however behind the scenes we have a scsi
> device with a request_queue. How this is used varies by class of driver
> (TYPE_DISK, TYPE_TYPE, etc). Care has to be taken into consideration of
> the fact that scsi drivers will probe asynchronously but the scsi-generic
> class_interface sg_add_device() will complete before. This means
> sd_probe() will use device_add_disk() for TYPE_DISK and have its
> debugfs_dir created *after* the scsi-generic device is created.
> 
> For scsi-generic then we symlink to the real debugfs_dir only during a
> blktrace ioctl, but we do this only once. We also have to special-case
> yet another solution for drivers which use the bsg queue.
> 
> This goes tested with:
> 
>   o nvme partitions
>   o ISCSI with tgt, and blktracing against scsi-generic with:
>     o block
>     o tape
>     o cdrom
>     o media changer
> 
> Screenshots of what the debugfs for block looks like after running
> blktrace on a system with sg0  which has a raid controllerand then sg1
> as the media changer:
> 
>  # ls -l /sys/kernel/debug/block
> total 0
> drwxr-xr-x  3 root root 0 May  9 02:31 bsg
> drwxr-xr-x 19 root root 0 May  9 02:31 nvme0n1
> drwxr-xr-x 19 root root 0 May  9 02:31 nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p1 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p2 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p3 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p5 -> nvme1n1
> lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p6 -> nvme1n1
> drwxr-xr-x  2 root root 0 May  9 02:33 sch0
> lrwxrwxrwx  1 root root 0 May  9 02:33 sg0 -> bsg/2:0:0:0
> lrwxrwxrwx  1 root root 0 May  9 02:33 sg1 -> sch0
> drwxr-xr-x  5 root root 0 May  9 02:31 vda
> lrwxrwxrwx  1 root root 0 May  9 02:31 vda1 -> vda
> 
> Code for handling the  ebugfs_dir did get more complicatd for
> scsi-generic but this is technical debt. For the other types of devices,
> this simplifies the code considerably, with the only penalty now being
> that we're always creating the request queue debugfs directory for the
> request-based block device drivers.
> 
> The symlink use also makes it clearer when the request_queue is shared.
> 
> This patch is part of the work which disputes the severity of
> CVE-2019-19770 which shows this issue is not a core debugfs issue, but
> a misuse of debugfs within blktace.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Omar Sandoval <osandov@fb.com>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Nicolai Stange <nstange@suse.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: yu kuai <yukuai3@huawei.com>
> Cc: Christof Schmitt <christof.schmitt@de.ibm.com>
> Reported-by: syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
> Fixes: 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>  block/blk-debugfs.c          | 187 +++++++++++++++++++++++++++++++++++
>  block/blk-mq-debugfs.c       |   5 -
>  block/blk-sysfs.c            |   3 +
>  block/blk.h                  |  16 +++
>  block/bsg.c                  |   2 +
>  block/partitions/core.c      |   9 ++
>  drivers/scsi/ch.c            |   1 +
>  drivers/scsi/sg.c            |  75 ++++++++++++++
>  drivers/scsi/st.c            |   2 +
>  include/linux/blkdev.h       |   4 +-
>  include/linux/blktrace_api.h |   1 -
>  include/linux/genhd.h        |  69 +++++++++++++
>  kernel/trace/blktrace.c      |  24 +++--
>  13 files changed, 385 insertions(+), 13 deletions(-)
> 
> diff --git a/block/blk-debugfs.c b/block/blk-debugfs.c
> index 19091e1effc0..d40f12aecf8a 100644
> --- a/block/blk-debugfs.c
> +++ b/block/blk-debugfs.c
> @@ -8,8 +8,195 @@
>  #include <linux/debugfs.h>
>  
>  struct dentry *blk_debugfs_root;
> +struct dentry *blk_debugfs_bsg = NULL;
> +
> +/**
> + * enum blk_debugfs_dir_type - block device debugfs directory type
> + * @BLK_DBG_DIR_BASE: the block device debugfs_dir exists on the base
> + * 	system <system-debugfs-dir>/block/ debugfs directory.
> + * @BLK_DBG_DIR_BSG: the block device debugfs_dir is under the directory
> + * 	<system-debugfs-dir>/block/bsg/
> + */
> +enum blk_debugfs_dir_type {
> +	BLK_DBG_DIR_BASE = 1,
> +	BLK_DBG_DIR_BSG,
> +};
>  
>  void blk_debugfs_register(void)
>  {
>  	blk_debugfs_root = debugfs_create_dir("block", NULL);
>  }
> +
> +static struct dentry *queue_get_base_dir(enum blk_debugfs_dir_type type)
> +{
> +	switch (type) {
> +	case BLK_DBG_DIR_BASE:
> +		return blk_debugfs_root;
> +	case BLK_DBG_DIR_BSG:
> +		return blk_debugfs_bsg;
> +	}
> +	return NULL;
> +}
> +
> +static void queue_debugfs_register_type(struct request_queue *q,
> +					const char *name,
> +					enum blk_debugfs_dir_type type)
> +{
> +	struct dentry *base_dir = queue_get_base_dir(type);
> +
> +	q->debugfs_dir = debugfs_create_dir(name, base_dir);
> +}
> +
> +/**
> + * blk_queue_debugfs_register - register the debugfs_dir for the block device
> + * @q: the associated request_queue of the block device
> + * @name: the name of the block device exposed
> + *
> + * This is used to create the debugfs_dir used by the block layer and blktrace.
> + * Drivers which use any of the *add_disk*() calls or variants have this called
> + * automatically for them. This directory is removed automatically on
> + * blk_release_queue() once the request_queue reference count reaches 0.
> + */
> +void blk_queue_debugfs_register(struct request_queue *q, const char *name)
> +{
> +	queue_debugfs_register_type(q, name, BLK_DBG_DIR_BASE);
> +}
> +EXPORT_SYMBOL_GPL(blk_queue_debugfs_register);
> +
> +/**
> + * blk_queue_debugfs_unregister - remove the debugfs_dir for the block device
> + * @q: the associated request_queue of the block device
> + *
> + * Removes the debugfs_dir for the request_queue on the associated block device.
> + * This is handled for you on blk_release_queue(), and that should only be
> + * called once.
> + *
> + * Since we don't care where the debugfs_dir was created this is used for all
> + * types of of enum blk_debugfs_dir_type.
> + */
> +void blk_queue_debugfs_unregister(struct request_queue *q)
> +{
> +	debugfs_remove_recursive(q->debugfs_dir);
> +}
> +
> +static struct dentry *queue_debugfs_symlink_type(struct request_queue *q,
> +						 const char *src,
> +						 const char *dst,
> +						 enum blk_debugfs_dir_type type)
> +{
> +	struct dentry *dentry = ERR_PTR(-EINVAL);
> +	char *dir_dst;
> +
> +	dir_dst = kzalloc(PATH_MAX, GFP_KERNEL);
> +	if (!dir_dst)
> +		return dentry;
> +
> +	switch (type) {
> +	case BLK_DBG_DIR_BASE:
> +		if (dst)
> +			snprintf(dir_dst, PATH_MAX, "%s", dst);
> +		else if (!IS_ERR_OR_NULL(q->debugfs_dir))
> +			snprintf(dir_dst, PATH_MAX, "%s",
> +				 q->debugfs_dir->d_name.name);

How can debugfs_dir be NULL/error here?

And grabbing the name of a debugfs file is sketchy, just use the name
that you think you already have, from the device, don't rely on debugfs
working here.

And why a symlink anyway?  THat's a new addition, what is going to work
with that in userspace?

> +#ifdef CONFIG_DEBUG_FS
> +	p->debugfs_sym = blk_queue_debugfs_symlink(disk->queue, dev_name(pdev),
> +						   disk->disk_name);
> +#endif

No need to #ifdef this, right?

I feel like this patch series keeps getting more complex and messier
over time :(

greg k-h

Re: [PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls

[Luis Chamberlain]

On Sat, May 09, 2020 at 06:09:38PM -0700, Bart Van Assche wrote:
> On 2020-05-08 20:10, Luis Chamberlain wrote:
> > @@ -493,6 +496,12 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
> >  	 */
> >  	strreplace(buts->name, '/', '_');
> >  
> > +	if (q->blk_trace) {
> > +		pr_warn("Concurrent blktraces are not allowed on %s\n",
> > +			buts->name);
> > +		return -EBUSY;
> > +	}
> > +
> >  	bt = kzalloc(sizeof(*bt), GFP_KERNEL);
> >  	if (!bt)
> >  		return -ENOMEM;
> 
> Is this really sufficient? Shouldn't concurrent do_blk_trace_setup()
> calls that refer to the same request queue be serialized to really
> prevent that debugfs attribute creation fails?

We'd have to add something like a linked list. Right now I'm just
clarifying things which were not clear before. What you describe is
a functional feature change. I'm just trying to fix a bug and clarify
limitations.

> How about using the block device name instead of the partition name in
> the error message since the concurrency context is the block device and
> not the partition?

blk device argument can be NULL here. sg-generic is one case.

 Luis

Re: [PATCH v4 1/5] block: revert back to synchronous request_queue removal

[Luis Chamberlain]

On Sun, May 10, 2020 at 08:20:58AM +0200, Greg KH wrote:
> On Sat, May 09, 2020 at 03:10:54AM +0000, Luis Chamberlain wrote:
> > Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
> > v4.12 moved the work behind blk_release_queue() into a workqueue after a
> > splat floated around which indicated some work on blk_release_queue()
> > could sleep in blk_exit_rl(). This splat would be possible when a driver
> > called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
> > as its final call) from an atomic context.
> > 
> > blk_put_queue() decrements the refcount for the request_queue kobject,
> > and upon reaching 0 blk_release_queue() is called. Although blk_exit_rl()
> > is now removed through commit db6d9952356 ("block: remove request_list code")
> > on v5.0, we reserve the right to be able to sleep within blk_release_queue()
> > context.
> > 
> > The last reference for the request_queue must not be called from atomic
> > context. *When* the last reference to the request_queue reaches 0 varies,
> > and so let's take the opportunity to document when that is expected to
> > happen and also document the context of the related calls as best as possible
> > so we can avoid future issues, and with the hopes that the synchronous
> > request_queue removal sticks.
> > 
> > We revert back to synchronous request_queue removal because asynchronous
> > removal creates a regression with expected userspace interaction with
> > several drivers. An example is when removing the loopback driver, one
> > uses ioctls from userspace to do so, but upon return and if successful,
> > one expects the device to be removed. Likewise if one races to add another
> > device the new one may not be added as it is still being removed. This was
> > expected behavior before and it now fails as the device is still present
> > and busy still. Moving to asynchronous request_queue removal could have
> > broken many scripts which relied on the removal to have been completed if
> > there was no error. Document this expectation as well so that this
> > doesn't regress userspace again.
> > 
> > Using asynchronous request_queue removal however has helped us find
> > other bugs. In the future we can test what could break with this
> > arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.
> 
> You are adding documenation and might_sleep() calls all over the place
> in here, making the "real" change in the patch hard to pick out.
> 
> How about you split this up into 3 patches, one for documentation, one
> for might_sleep() and one for the real change?  Or maybe just 2 patches,
> but what you have here seems excessive.

Sure.

  Luis

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[Luis Chamberlain]

On Sat, May 09, 2020 at 05:58:55PM -0700, Bart Van Assche wrote:
> On 2020-05-08 20:10, Luis Chamberlain wrote:
> > Screenshots of what the debugfs for block looks like after running
> > blktrace on a system with sg0  which has a raid controllerand then sg1
> > as the media changer:
> > 
> >  # ls -l /sys/kernel/debug/block
> > total 0
> > drwxr-xr-x  3 root root 0 May  9 02:31 bsg
> > drwxr-xr-x 19 root root 0 May  9 02:31 nvme0n1
> > drwxr-xr-x 19 root root 0 May  9 02:31 nvme1n1
> > lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p1 -> nvme1n1
> > lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p2 -> nvme1n1
> > lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p3 -> nvme1n1
> > lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p5 -> nvme1n1
> > lrwxrwxrwx  1 root root 0 May  9 02:31 nvme1n1p6 -> nvme1n1
> > drwxr-xr-x  2 root root 0 May  9 02:33 sch0
> > lrwxrwxrwx  1 root root 0 May  9 02:33 sg0 -> bsg/2:0:0:0
> > lrwxrwxrwx  1 root root 0 May  9 02:33 sg1 -> sch0
> > drwxr-xr-x  5 root root 0 May  9 02:31 vda
> > lrwxrwxrwx  1 root root 0 May  9 02:31 vda1 -> vda
> 
> So this patch creates one soft link per partition at partition creation
> time instead of letting the blktrace code create one directory per
> partition when tracing starts?

Yes.

> Does this break running blktrace
> simultaneously for a partition and for the entire block device?

blktrace already has this limitation, one blktrace is only allowed per
request_queue, the next patch clarifies this, as we currently just error
out without telling the user what has happened.e user what has
happened.e user what has happened.e user what has happened.

> > +static struct dentry *queue_debugfs_symlink_type(struct request_queue *q,
> > +						 const char *src,
> > +						 const char *dst,
> > +						 enum blk_debugfs_dir_type type)
> > +{
> > +	struct dentry *dentry = ERR_PTR(-EINVAL);
> > +	char *dir_dst;
> > +
> > +	dir_dst = kzalloc(PATH_MAX, GFP_KERNEL);
> > +	if (!dir_dst)
> > +		return dentry;
> > +
> > +	switch (type) {
> > +	case BLK_DBG_DIR_BASE:
> > +		if (dst)
> > +			snprintf(dir_dst, PATH_MAX, "%s", dst);
> > +		else if (!IS_ERR_OR_NULL(q->debugfs_dir))
> > +			snprintf(dir_dst, PATH_MAX, "%s",
> > +				 q->debugfs_dir->d_name.name);
> > +		else
> > +			goto out;
> > +		break;
> > +	case BLK_DBG_DIR_BSG:
> > +		if (dst)
> > +			snprintf(dir_dst, PATH_MAX, "bsg/%s", dst);
> > +		else
> > +			goto out;
> > +		break;
> > +	}
> > +
> > +	/*
> > +	 * The base block debugfs directory is always used for the symlinks,
> > +	 * their target is what changes.
> > +	 */
> > +	dentry = debugfs_create_symlink(src, blk_debugfs_root, dir_dst);
> > +out:
> > +	kfree(dir_dst);
> > +
> > +	return dentry;
> > +}
> 
> Please use kasprintf() instead of k?alloc() followed by snprintf().

Sure thing.

  Luis

Re: [PATCH v4 3/5] blktrace: fix debugfs use after free

[Luis Chamberlain]

On Sun, May 10, 2020 at 08:26:36AM +0200, Greg KH wrote:
> On Sat, May 09, 2020 at 03:10:56AM +0000, Luis Chamberlain wrote:
> > diff --git a/block/blk-debugfs.c b/block/blk-debugfs.c
> > index 19091e1effc0..d40f12aecf8a 100644
> > --- a/block/blk-debugfs.c
> > +++ b/block/blk-debugfs.c
> > +static struct dentry *queue_debugfs_symlink_type(struct request_queue *q,
> > +						 const char *src,
> > +						 const char *dst,
> > +						 enum blk_debugfs_dir_type type)
> > +{
> > +	struct dentry *dentry = ERR_PTR(-EINVAL);
> > +	char *dir_dst;
> > +
> > +	dir_dst = kzalloc(PATH_MAX, GFP_KERNEL);
> > +	if (!dir_dst)
> > +		return dentry;
> > +
> > +	switch (type) {
> > +	case BLK_DBG_DIR_BASE:
> > +		if (dst)
> > +			snprintf(dir_dst, PATH_MAX, "%s", dst);
> > +		else if (!IS_ERR_OR_NULL(q->debugfs_dir))
> > +			snprintf(dir_dst, PATH_MAX, "%s",
> > +				 q->debugfs_dir->d_name.name);
> 
> How can debugfs_dir be NULL/error here?

If someone were to move blk_queue_debugfs_symlink() to sg_add_device(),
this can happen as the sd_probe() will run asynchronously. The comment
in blk_queue_debugfs_symlink() suggest to be mindful of async probes.

I can do-away with this *iff* we add an probe_complete call to the
driver core class interface.

> And grabbing the name of a debugfs file is sketchy, just use the name
> that you think you already have, from the device, don't rely on debugfs
> working here.
> 
> And why a symlink anyway?

As the commit log explains, and the comment to the calling functions
explains, we have a shared request_queue, and this has been the way,
the symlink makes that sharing explicit. The next patch explains why
using two blktraces on two separate devices would fail, the symlink
makes this sharing quite obvious.

It also simplifies the trace/kernel/blktrace.c code to just use the
debugfs_dir only.

> THat's a new addition, what is going to work
> with that in userspace?

This already works in userspace with blktrace. If you see a regression,
please let me know.

> > +#ifdef CONFIG_DEBUG_FS
> > +	p->debugfs_sym = blk_queue_debugfs_symlink(disk->queue, dev_name(pdev),
> > +						   disk->disk_name);
> > +#endif
> 
> No need to #ifdef this, right?

It is needed because of the p->debugfs_sym.

> I feel like this patch series keeps getting more complex and messier
> over time :(

I'm afraid that use of blktrace in scsi-generic is the culprit, the
technical debt of the complexities around how the request_queue is
shared and properly providing backward compatibilty by removing the
debufs_looksup() has caught up to us.

If we were to agree to remove scsi-generic support from blktrace first,
that would simplify things hugely.

  Luis

Re: [PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls

[Luis Chamberlain]

On Mon, May 11, 2020 at 7:39 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> On Sat, May 09, 2020 at 06:09:38PM -0700, Bart Van Assche wrote:
> > How about using the block device name instead of the partition name in
> > the error message since the concurrency context is the block device and
> > not the partition?
>
> blk device argument can be NULL here. sg-generic is one case.

I'm going to add a comment about this, as it is easily forgotten.

  Luis

Re: [PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls

[Luis Chamberlain]

On Mon, May 11, 2020 at 7:39 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> On Sat, May 09, 2020 at 06:09:38PM -0700, Bart Van Assche wrote:
> > How about using the block device name instead of the partition name in
> > the error message since the concurrency context is the block device and
> > not the partition?
>
> blk device argument can be NULL here. sg-generic is one case.

I'm going to add a comment about this, as it is easily forgotten.

  Luis