[PATCH 0/5] block subsystem refcounter conversions

[PATCH 0/5] block subsystem refcounter conversions

[Elena Reshetova]

Now when new refcount_t type and API are finally merged
(see include/linux/refcount.h), the following
patches convert various refcounters in the block susystem from atomic_t
to refcount_t. By doing this we prevent intentional or accidental
underflows or overflows that can led to use-after-free vulnerabilities.

The below patches are fully independent and can be cherry-picked separately.
Since we convert all kernel subsystems in the same fashion, resulting
in about 300 patches, we have to group them for sending at least in some
fashion to be manageable. Please excuse the long cc list.

Elena Reshetova (5):
  block: convert bio.__bi_cnt from atomic_t to refcount_t
  block: convert blk_queue_tag.refcnt from atomic_t to refcount_t
  block: convert blkcg_gq.refcnt from atomic_t to refcount_t
  block: convert io_context.active_ref from atomic_t to refcount_t
  block: convert bsg_device.ref_count from atomic_t to refcount_t

 block/bio.c                |  6 +++---
 block/blk-cgroup.c         |  2 +-
 block/blk-ioc.c            |  4 ++--
 block/blk-tag.c            |  8 ++++----
 block/bsg.c                |  9 +++++----
 block/cfq-iosched.c        |  4 ++--
 fs/btrfs/volumes.c         |  2 +-
 include/linux/bio.h        |  4 ++--
 include/linux/blk-cgroup.h | 11 ++++++-----
 include/linux/blk_types.h  |  3 ++-
 include/linux/blkdev.h     |  3 ++-
 include/linux/iocontext.h  |  7 ++++---
 12 files changed, 34 insertions(+), 29 deletions(-)

-- 
2.7.4

[PATCH 1/5] block: convert bio.__bi_cnt from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 block/bio.c               | 6 +++---
 fs/btrfs/volumes.c        | 2 +-
 include/linux/bio.h       | 4 ++--
 include/linux/blk_types.h | 3 ++-
 4 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 5eec5e0..3dffc17 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -275,7 +275,7 @@ void bio_init(struct bio *bio, struct bio_vec *table,
 {
 	memset(bio, 0, sizeof(*bio));
 	atomic_set(&bio->__bi_remaining, 1);
-	atomic_set(&bio->__bi_cnt, 1);
+	refcount_set(&bio->__bi_cnt, 1);
 
 	bio->bi_io_vec = table;
 	bio->bi_max_vecs = max_vecs;
@@ -543,12 +543,12 @@ void bio_put(struct bio *bio)
 	if (!bio_flagged(bio, BIO_REFFED))
 		bio_free(bio);
 	else {
-		BIO_BUG_ON(!atomic_read(&bio->__bi_cnt));
+		BIO_BUG_ON(!refcount_read(&bio->__bi_cnt));
 
 		/*
 		 * last put frees it
 		 */
-		if (atomic_dec_and_test(&bio->__bi_cnt))
+		if (refcount_dec_and_test(&bio->__bi_cnt))
 			bio_free(bio);
 	}
 }
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 13e55d1..ff1fe9d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -441,7 +441,7 @@ static noinline void run_scheduled_bios(struct btrfs_device *device)
 		    waitqueue_active(&fs_info->async_submit_wait))
 			wake_up(&fs_info->async_submit_wait);
 
-		BUG_ON(atomic_read(&cur->__bi_cnt) == 0);
+		BUG_ON(refcount_read(&cur->__bi_cnt) == 0);
 
 		/*
 		 * if we're doing the sync list, record that our
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 8e52119..44ac678 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -234,7 +234,7 @@ static inline void bio_get(struct bio *bio)
 {
 	bio->bi_flags |= (1 << BIO_REFFED);
 	smp_mb__before_atomic();
-	atomic_inc(&bio->__bi_cnt);
+	refcount_inc(&bio->__bi_cnt);
 }
 
 static inline void bio_cnt_set(struct bio *bio, unsigned int count)
@@ -243,7 +243,7 @@ static inline void bio_cnt_set(struct bio *bio, unsigned int count)
 		bio->bi_flags |= (1 << BIO_REFFED);
 		smp_mb__before_atomic();
 	}
-	atomic_set(&bio->__bi_cnt, count);
+	refcount_set(&bio->__bi_cnt, count);
 }
 
 static inline bool bio_flagged(struct bio *bio, unsigned int bit)
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index d703acb..c41407f 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -7,6 +7,7 @@
 
 #include <linux/types.h>
 #include <linux/bvec.h>
+#include <linux/refcount.h>
 
 struct bio_set;
 struct bio;
@@ -73,7 +74,7 @@ struct bio {
 
 	unsigned short		bi_max_vecs;	/* max bvl_vecs we can hold */
 
-	atomic_t		__bi_cnt;	/* pin count */
+	refcount_t		__bi_cnt;	/* pin count */
 
 	struct bio_vec		*bi_io_vec;	/* the actual vec list */
 
-- 
2.7.4

[PATCH 2/5] block: convert blk_queue_tag.refcnt from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 block/blk-tag.c        | 8 ++++----
 include/linux/blkdev.h | 3 ++-
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/block/blk-tag.c b/block/blk-tag.c
index 07cc329..d83555e 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -35,7 +35,7 @@ EXPORT_SYMBOL(blk_queue_find_tag);
  */
 void blk_free_tags(struct blk_queue_tag *bqt)
 {
-	if (atomic_dec_and_test(&bqt->refcnt)) {
+	if (refcount_dec_and_test(&bqt->refcnt)) {
 		BUG_ON(find_first_bit(bqt->tag_map, bqt->max_depth) <
 							bqt->max_depth);
 
@@ -130,7 +130,7 @@ static struct blk_queue_tag *__blk_queue_init_tags(struct request_queue *q,
 	if (init_tag_map(q, tags, depth))
 		goto fail;
 
-	atomic_set(&tags->refcnt, 1);
+	refcount_set(&tags->refcnt, 1);
 	tags->alloc_policy = alloc_policy;
 	tags->next_tag = 0;
 	return tags;
@@ -180,7 +180,7 @@ int blk_queue_init_tags(struct request_queue *q, int depth,
 		queue_flag_set(QUEUE_FLAG_QUEUED, q);
 		return 0;
 	} else
-		atomic_inc(&tags->refcnt);
+		refcount_inc(&tags->refcnt);
 
 	/*
 	 * assign it, all done
@@ -225,7 +225,7 @@ int blk_queue_resize_tags(struct request_queue *q, int new_depth)
 	 * Currently cannot replace a shared tag map with a new
 	 * one, so error out if this is the case
 	 */
-	if (atomic_read(&bqt->refcnt) != 1)
+	if (refcount_read(&bqt->refcnt) != 1)
 		return -EBUSY;
 
 	/*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index aecca0e..95ef11c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -25,6 +25,7 @@
 #include <linux/percpu-refcount.h>
 #include <linux/scatterlist.h>
 #include <linux/blkzoned.h>
+#include <linux/refcount.h>
 
 struct module;
 struct scsi_ioctl_command;
@@ -288,7 +289,7 @@ struct blk_queue_tag {
 	unsigned long *tag_map;		/* bit map of free/busy tags */
 	int max_depth;			/* what we will send to device */
 	int real_max_depth;		/* what the array can hold */
-	atomic_t refcnt;		/* map can be shared */
+	refcount_t refcnt;		/* map can be shared */
 	int alloc_policy;		/* tag allocation policy */
 	int next_tag;			/* next tag */
 };
-- 
2.7.4

[PATCH 3/5] block: convert blkcg_gq.refcnt from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 block/blk-cgroup.c         |  2 +-
 include/linux/blk-cgroup.h | 11 ++++++-----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 295e98c2..75de844 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -106,7 +106,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
 	blkg->q = q;
 	INIT_LIST_HEAD(&blkg->q_node);
 	blkg->blkcg = blkcg;
-	atomic_set(&blkg->refcnt, 1);
+	refcount_set(&blkg->refcnt, 1);
 
 	/* root blkg uses @q->root_rl, init rl only for !root blkgs */
 	if (blkcg != &blkcg_root) {
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 01b62e7..0d3efa9 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -19,6 +19,7 @@
 #include <linux/radix-tree.h>
 #include <linux/blkdev.h>
 #include <linux/atomic.h>
+#include <linux/refcount.h>
 
 /* percpu_counter batch for blkg_[rw]stats, per-cpu drift doesn't matter */
 #define BLKG_STAT_CPU_BATCH	(INT_MAX / 2)
@@ -122,7 +123,7 @@ struct blkcg_gq {
 	struct request_list		rl;
 
 	/* reference count */
-	atomic_t			refcnt;
+	refcount_t			refcnt;
 
 	/* is this blkg online? protected by both blkcg and q locks */
 	bool				online;
@@ -354,8 +355,8 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
  */
 static inline void blkg_get(struct blkcg_gq *blkg)
 {
-	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
-	atomic_inc(&blkg->refcnt);
+	WARN_ON_ONCE(refcount_read(&blkg->refcnt) == 0);
+	refcount_inc(&blkg->refcnt);
 }
 
 void __blkg_release_rcu(struct rcu_head *rcu);
@@ -366,8 +367,8 @@ void __blkg_release_rcu(struct rcu_head *rcu);
  */
 static inline void blkg_put(struct blkcg_gq *blkg)
 {
-	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
-	if (atomic_dec_and_test(&blkg->refcnt))
+	WARN_ON_ONCE(refcount_read(&blkg->refcnt) == 0);
+	if (refcount_dec_and_test(&blkg->refcnt))
 		call_rcu(&blkg->rcu_head, __blkg_release_rcu);
 }
 
-- 
2.7.4

[PATCH 4/5] block: convert io_context.active_ref from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 block/blk-ioc.c           | 4 ++--
 block/cfq-iosched.c       | 4 ++--
 include/linux/iocontext.h | 7 ++++---
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index b12f9c8..130dc23 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -173,7 +173,7 @@ void put_io_context_active(struct io_context *ioc)
 	unsigned long flags;
 	struct io_cq *icq;
 
-	if (!atomic_dec_and_test(&ioc->active_ref)) {
+	if (!refcount_dec_and_test(&ioc->active_ref)) {
 		put_io_context(ioc);
 		return;
 	}
@@ -256,7 +256,7 @@ int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
 	/* initialize */
 	atomic_long_set(&ioc->refcount, 1);
 	atomic_set(&ioc->nr_tasks, 1);
-	atomic_set(&ioc->active_ref, 1);
+	refcount_set(&ioc->active_ref, 1);
 	spin_lock_init(&ioc->lock);
 	INIT_RADIX_TREE(&ioc->icq_tree, GFP_ATOMIC | __GFP_HIGH);
 	INIT_HLIST_HEAD(&ioc->icq_list);
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 9212627..2871bb9 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2959,7 +2959,7 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
 	 * task has exited, don't wait
 	 */
 	cic = cfqd->active_cic;
-	if (!cic || !atomic_read(&cic->icq.ioc->active_ref))
+	if (!cic || !refcount_read(&cic->icq.ioc->active_ref))
 		return;
 
 	/*
@@ -3959,7 +3959,7 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 
 	if (cfqq->next_rq && req_noidle(cfqq->next_rq))
 		enable_idle = 0;
-	else if (!atomic_read(&cic->icq.ioc->active_ref) ||
+	else if (!refcount_read(&cic->icq.ioc->active_ref) ||
 		 !cfqd->cfq_slice_idle ||
 		 (!cfq_cfqq_deep(cfqq) && CFQQ_SEEKY(cfqq)))
 		enable_idle = 0;
diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index df38db2..a1e28c3 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -3,6 +3,7 @@
 
 #include <linux/radix-tree.h>
 #include <linux/rcupdate.h>
+#include <linux/refcount.h>
 #include <linux/workqueue.h>
 
 enum {
@@ -96,7 +97,7 @@ struct io_cq {
  */
 struct io_context {
 	atomic_long_t refcount;
-	atomic_t active_ref;
+	refcount_t active_ref;
 	atomic_t nr_tasks;
 
 	/* all the fields below are protected by this lock */
@@ -128,9 +129,9 @@ struct io_context {
 static inline void get_io_context_active(struct io_context *ioc)
 {
 	WARN_ON_ONCE(atomic_long_read(&ioc->refcount) <= 0);
-	WARN_ON_ONCE(atomic_read(&ioc->active_ref) <= 0);
+	WARN_ON_ONCE(refcount_read(&ioc->active_ref) == 0);
 	atomic_long_inc(&ioc->refcount);
-	atomic_inc(&ioc->active_ref);
+	refcount_inc(&ioc->active_ref);
 }
 
 static inline void ioc_task_link(struct io_context *ioc)
-- 
2.7.4

[PATCH 5/5] block: convert bsg_device.ref_count from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 block/bsg.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/bsg.c b/block/bsg.c
index 74835db..6d0ceb9 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -21,6 +21,7 @@
 #include <linux/idr.h>
 #include <linux/bsg.h>
 #include <linux/slab.h>
+#include <linux/refcount.h>
 
 #include <scsi/scsi.h>
 #include <scsi/scsi_ioctl.h>
@@ -38,7 +39,7 @@ struct bsg_device {
 	struct list_head busy_list;
 	struct list_head done_list;
 	struct hlist_node dev_list;
-	atomic_t ref_count;
+	refcount_t ref_count;
 	int queued_cmds;
 	int done_cmds;
 	wait_queue_head_t wq_done;
@@ -711,7 +712,7 @@ static int bsg_put_device(struct bsg_device *bd)
 
 	mutex_lock(&bsg_mutex);
 
-	do_free = atomic_dec_and_test(&bd->ref_count);
+	do_free = refcount_dec_and_test(&bd->ref_count);
 	if (!do_free) {
 		mutex_unlock(&bsg_mutex);
 		goto out;
@@ -763,7 +764,7 @@ static struct bsg_device *bsg_add_device(struct inode *inode,
 
 	bsg_set_block(bd, file);
 
-	atomic_set(&bd->ref_count, 1);
+	refcount_set(&bd->ref_count, 1);
 	mutex_lock(&bsg_mutex);
 	hlist_add_head(&bd->dev_list, bsg_dev_idx_hash(iminor(inode)));
 
@@ -783,7 +784,7 @@ static struct bsg_device *__bsg_get_device(int minor, struct request_queue *q)
 
 	hlist_for_each_entry(bd, bsg_dev_idx_hash(minor), dev_list) {
 		if (bd->queue == q) {
-			atomic_inc(&bd->ref_count);
+			refcount_inc(&bd->ref_count);
 			goto found;
 		}
 	}
-- 
2.7.4

Re: [PATCH 0/5] block subsystem refcounter conversions

[Jens Axboe]

On 02/20/2017 04:16 AM, Elena Reshetova wrote:
> Now when new refcount_t type and API are finally merged
> (see include/linux/refcount.h), the following
> patches convert various refcounters in the block susystem from atomic_t
> to refcount_t. By doing this we prevent intentional or accidental
> underflows or overflows that can led to use-after-free vulnerabilities.
> 
> The below patches are fully independent and can be cherry-picked separately.
> Since we convert all kernel subsystems in the same fashion, resulting
> in about 300 patches, we have to group them for sending at least in some
> fashion to be manageable. Please excuse the long cc list.
> 
> Elena Reshetova (5):
>   block: convert bio.__bi_cnt from atomic_t to refcount_t
>   block: convert blk_queue_tag.refcnt from atomic_t to refcount_t
>   block: convert blkcg_gq.refcnt from atomic_t to refcount_t
>   block: convert io_context.active_ref from atomic_t to refcount_t
>   block: convert bsg_device.ref_count from atomic_t to refcount_t

I went to look at the implementation, and the size of a refcount_t.
But the code is not there?! You say it's finally merged, where is
it merged? Don't send code like this without the necessary
infrastructure being in mainline.

-- 
Jens Axboe

Re: [PATCH 0/5] block subsystem refcounter conversions

[James Bottomley]

On Mon, 2017-02-20 at 08:15 -0700, Jens Axboe wrote:
> On 02/20/2017 04:16 AM, Elena Reshetova wrote:
> > Now when new refcount_t type and API are finally merged
> > (see include/linux/refcount.h), the following
> > patches convert various refcounters in the block susystem from 
> > atomic_t to refcount_t. By doing this we prevent intentional or 
> > accidental underflows or overflows that can led to use-after-free
> > vulnerabilities.

This description isn't right ... nothing is prevented; we get warnings
on saturation and use after free with this.

> > The below patches are fully independent and can be cherry-picked 
> > separately. Since we convert all kernel subsystems in the same 
> > fashion, resulting in about 300 patches, we have to group them for 
> > sending at least in some fashion to be manageable. Please excuse
> > the long cc list.
> > 
> > Elena Reshetova (5):
> >   block: convert bio.__bi_cnt from atomic_t to refcount_t
> >   block: convert blk_queue_tag.refcnt from atomic_t to refcount_t
> >   block: convert blkcg_gq.refcnt from atomic_t to refcount_t
> >   block: convert io_context.active_ref from atomic_t to refcount_t
> >   block: convert bsg_device.ref_count from atomic_t to refcount_t
> 
> I went to look at the implementation, and the size of a refcount_t.
> But the code is not there?! You say it's finally merged, where is
> it merged? Don't send code like this without the necessary
> infrastructure being in mainline.

It's one of those no discussion except notice by tipbot things because
Ingo liked it.

The size is atomic_t, but the primitives check for overflow and check
inc from zero and things, so in a true refcounting situation we get
automatic warnings of problems inside the primitives.

That said, if we were going to convert the block layer to this
semantic, surely the benefit of the conversion would be getting rid of
the now unnecessary BUG_ON and WARN_ON in the code, which do exactly
the same thing the refcount primitives now do?

James

Re: [PATCH 0/5] block subsystem refcounter conversions

[Jens Axboe]

On 02/20/2017 08:41 AM, James Bottomley wrote:
> On Mon, 2017-02-20 at 08:15 -0700, Jens Axboe wrote:
>> On 02/20/2017 04:16 AM, Elena Reshetova wrote:
>>> Now when new refcount_t type and API are finally merged
>>> (see include/linux/refcount.h), the following
>>> patches convert various refcounters in the block susystem from 
>>> atomic_t to refcount_t. By doing this we prevent intentional or 
>>> accidental underflows or overflows that can led to use-after-free
>>> vulnerabilities.
> 
> This description isn't right ... nothing is prevented; we get warnings
> on saturation and use after free with this.
> 
>>> The below patches are fully independent and can be cherry-picked 
>>> separately. Since we convert all kernel subsystems in the same 
>>> fashion, resulting in about 300 patches, we have to group them for 
>>> sending at least in some fashion to be manageable. Please excuse
>>> the long cc list.
>>>
>>> Elena Reshetova (5):
>>>   block: convert bio.__bi_cnt from atomic_t to refcount_t
>>>   block: convert blk_queue_tag.refcnt from atomic_t to refcount_t
>>>   block: convert blkcg_gq.refcnt from atomic_t to refcount_t
>>>   block: convert io_context.active_ref from atomic_t to refcount_t
>>>   block: convert bsg_device.ref_count from atomic_t to refcount_t
>>
>> I went to look at the implementation, and the size of a refcount_t.
>> But the code is not there?! You say it's finally merged, where is
>> it merged? Don't send code like this without the necessary
>> infrastructure being in mainline.
> 
> It's one of those no discussion except notice by tipbot things because
> Ingo liked it.
> 
> The size is atomic_t, but the primitives check for overflow and check
> inc from zero and things, so in a true refcounting situation we get
> automatic warnings of problems inside the primitives.
> 
> That said, if we were going to convert the block layer to this
> semantic, surely the benefit of the conversion would be getting rid of
> the now unnecessary BUG_ON and WARN_ON in the code, which do exactly
> the same thing the refcount primitives now do?

Well, I have no idea what it does, which is why I went to look at the
code. So any talk of converting the block layer is premature.  But it's
not there. I'll defer judgment until we have something in mainline,
until then I've archived this thread.

And I agree, keeping warn/bug for cases that should be handled
with this framework would be counter productive and pointless.

-- 
Jens Axboe

Re: [PATCH 0/5] block subsystem refcounter conversions

[Peter Zijlstra]

On Mon, Feb 20, 2017 at 07:41:01AM -0800, James Bottomley wrote:
> On Mon, 2017-02-20 at 08:15 -0700, Jens Axboe wrote:
> > On 02/20/2017 04:16 AM, Elena Reshetova wrote:
> > > Now when new refcount_t type and API are finally merged
> > > (see include/linux/refcount.h), the following
> > > patches convert various refcounters in the block susystem from 
> > > atomic_t to refcount_t. By doing this we prevent intentional or 
> > > accidental underflows or overflows that can led to use-after-free
> > > vulnerabilities.
> 
> This description isn't right ... nothing is prevented; we get warnings
> on saturation and use after free with this.

The thing that is prevented is overflow and then a use-after-free by
making it a leak.

Modular stuff, you put and free at: (n+1) mod n, by saturating at n-1
we'll never get there.

So you loose use-after-free, you gain a resource leak. The general idea
being that use-after-free is a nice trampoline for exploits, leaks are
'only' a DoS.

Re: [PATCH 0/5] block subsystem refcounter conversions

[James Bottomley]

On Mon, 2017-02-20 at 17:56 +0100, Peter Zijlstra wrote:
> On Mon, Feb 20, 2017 at 07:41:01AM -0800, James Bottomley wrote:
> > On Mon, 2017-02-20 at 08:15 -0700, Jens Axboe wrote:
> > > On 02/20/2017 04:16 AM, Elena Reshetova wrote:
> > > > Now when new refcount_t type and API are finally merged
> > > > (see include/linux/refcount.h), the following
> > > > patches convert various refcounters in the block susystem from 
> > > > atomic_t to refcount_t. By doing this we prevent intentional or
> > > > accidental underflows or overflows that can led to use-after
> > > > -free vulnerabilities.
> > 
> > This description isn't right ... nothing is prevented; we get 
> > warnings on saturation and use after free with this.
> 
> The thing that is prevented is overflow and then a use-after-free by
> making it a leak.
> 
> Modular stuff, you put and free at: (n+1) mod n, by saturating at n-1
> we'll never get there.
> 
> So you loose use-after-free, you gain a resource leak. The general 
> idea being that use-after-free is a nice trampoline for exploits, 
> leaks are 'only' a DoS.

OK, I see the intention: it's protection from outside influence.  It
still doesn't prevent *us* from screwing up in the kernel and inducing
a use after free by doing too many puts (or too few gets) ... that's
what the message suggests to me (me coding wrongly is accidental
underflows or overflows as I read it).

James