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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org, Theodore Tso <tytso@mit.edu>,
	Chris Mason <clm@fb.com>, Tejun Heo <tj@kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 18/24] memcg: fix a crash in wb_workfn when a device disappears
Date: Thu, 11 Feb 2021 16:02:52 +0100
Message-ID: <20210211150148.555189808@linuxfoundation.org> (raw)
In-Reply-To: <20210211150147.743660073@linuxfoundation.org>

From: Theodore Ts'o <tytso@mit.edu>

[ Upstream commit 68f23b89067fdf187763e75a56087550624fdbee ]

Without memcg, there is a one-to-one mapping between the bdi and
bdi_writeback structures.  In this world, things are fairly
straightforward; the first thing bdi_unregister() does is to shutdown
the bdi_writeback structure (or wb), and part of that writeback ensures
that no other work queued against the wb, and that the wb is fully
drained.

With memcg, however, there is a one-to-many relationship between the bdi
and bdi_writeback structures; that is, there are multiple wb objects
which can all point to a single bdi.  There is a refcount which prevents
the bdi object from being released (and hence, unregistered).  So in
theory, the bdi_unregister() *should* only get called once its refcount
goes to zero (bdi_put will drop the refcount, and when it is zero,
release_bdi gets called, which calls bdi_unregister).

Unfortunately, del_gendisk() in block/gen_hd.c never got the memo about
the Brave New memcg World, and calls bdi_unregister directly.  It does
this without informing the file system, or the memcg code, or anything
else.  This causes the root wb associated with the bdi to be
unregistered, but none of the memcg-specific wb's are shutdown.  So when
one of these wb's are woken up to do delayed work, they try to
dereference their wb->bdi->dev to fetch the device name, but
unfortunately bdi->dev is now NULL, thanks to the bdi_unregister()
called by del_gendisk().  As a result, *boom*.

Fortunately, it looks like the rest of the writeback path is perfectly
happy with bdi->dev and bdi->owner being NULL, so the simplest fix is to
create a bdi_dev_name() function which can handle bdi->dev being NULL.
This also allows us to bulletproof the writeback tracepoints to prevent
them from dereferencing a NULL pointer and crashing the kernel if one is
tracing with memcg's enabled, and an iSCSI device dies or a USB storage
stick is pulled.

The most common way of triggering this will be hotremoval of a device
while writeback with memcg enabled is going on.  It was triggering
several times a day in a heavily loaded production environment.

Google Bug Id: 145475544

Link: https://lore.kernel.org/r/20191227194829.150110-1-tytso@mit.edu
Link: http://lkml.kernel.org/r/20191228005211.163952-1-tytso@mit.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Chris Mason <clm@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/fs-writeback.c                |  2 +-
 include/linux/backing-dev.h      | 10 ++++++++++
 include/trace/events/writeback.h | 29 +++++++++++++----------------
 mm/backing-dev.c                 |  1 +
 4 files changed, 25 insertions(+), 17 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index f2d0c4acb3cbb..a247cb4b00e2d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1986,7 +1986,7 @@ void wb_workfn(struct work_struct *work)
 						struct bdi_writeback, dwork);
 	long pages_written;
 
-	set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
+	set_worker_desc("flush-%s", bdi_dev_name(wb->bdi));
 	current->flags |= PF_SWAPWRITE;
 
 	if (likely(!current_is_workqueue_rescuer() ||
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index c28a47cbe355e..1ef4aca7b953f 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -13,6 +13,7 @@
 #include <linux/fs.h>
 #include <linux/sched.h>
 #include <linux/blkdev.h>
+#include <linux/device.h>
 #include <linux/writeback.h>
 #include <linux/blk-cgroup.h>
 #include <linux/backing-dev-defs.h>
@@ -498,4 +499,13 @@ static inline int bdi_rw_congested(struct backing_dev_info *bdi)
 				  (1 << WB_async_congested));
 }
 
+extern const char *bdi_unknown_name;
+
+static inline const char *bdi_dev_name(struct backing_dev_info *bdi)
+{
+	if (!bdi || !bdi->dev)
+		return bdi_unknown_name;
+	return dev_name(bdi->dev);
+}
+
 #endif	/* _LINUX_BACKING_DEV_H */
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index b463e2575117e..300afa559f467 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -66,8 +66,8 @@ TRACE_EVENT(writeback_dirty_page,
 
 	TP_fast_assign(
 		strscpy_pad(__entry->name,
-			    mapping ? dev_name(inode_to_bdi(mapping->host)->dev) : "(unknown)",
-			    32);
+			    bdi_dev_name(mapping ? inode_to_bdi(mapping->host) :
+					 NULL), 32);
 		__entry->ino = mapping ? mapping->host->i_ino : 0;
 		__entry->index = page->index;
 	),
@@ -96,8 +96,7 @@ DECLARE_EVENT_CLASS(writeback_dirty_inode_template,
 		struct backing_dev_info *bdi = inode_to_bdi(inode);
 
 		/* may be called for files on pseudo FSes w/ unregistered bdi */
-		strscpy_pad(__entry->name,
-			    bdi->dev ? dev_name(bdi->dev) : "(unknown)", 32);
+		strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->state		= inode->i_state;
 		__entry->flags		= flags;
@@ -177,7 +176,7 @@ DECLARE_EVENT_CLASS(writeback_write_inode_template,
 
 	TP_fast_assign(
 		strscpy_pad(__entry->name,
-			    dev_name(inode_to_bdi(inode)->dev), 32);
+			    bdi_dev_name(inode_to_bdi(inode)), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->sync_mode	= wbc->sync_mode;
 		__entry->cgroup_ino	= __trace_wbc_assign_cgroup(wbc);
@@ -220,9 +219,7 @@ DECLARE_EVENT_CLASS(writeback_work_class,
 		__field(unsigned int, cgroup_ino)
 	),
 	TP_fast_assign(
-		strscpy_pad(__entry->name,
-			    wb->bdi->dev ? dev_name(wb->bdi->dev) :
-			    "(unknown)", 32);
+		strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
 		__entry->nr_pages = work->nr_pages;
 		__entry->sb_dev = work->sb ? work->sb->s_dev : 0;
 		__entry->sync_mode = work->sync_mode;
@@ -275,7 +272,7 @@ DECLARE_EVENT_CLASS(writeback_class,
 		__field(unsigned int, cgroup_ino)
 	),
 	TP_fast_assign(
-		strscpy_pad(__entry->name, dev_name(wb->bdi->dev), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
 		__entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
 	),
 	TP_printk("bdi %s: cgroup_ino=%u",
@@ -297,7 +294,7 @@ TRACE_EVENT(writeback_bdi_register,
 		__array(char, name, 32)
 	),
 	TP_fast_assign(
-		strscpy_pad(__entry->name, dev_name(bdi->dev), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
 	),
 	TP_printk("bdi %s",
 		__entry->name
@@ -322,7 +319,7 @@ DECLARE_EVENT_CLASS(wbc_class,
 	),
 
 	TP_fast_assign(
-		strscpy_pad(__entry->name, dev_name(bdi->dev), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
 		__entry->nr_to_write	= wbc->nr_to_write;
 		__entry->pages_skipped	= wbc->pages_skipped;
 		__entry->sync_mode	= wbc->sync_mode;
@@ -373,7 +370,7 @@ TRACE_EVENT(writeback_queue_io,
 		__field(unsigned int,	cgroup_ino)
 	),
 	TP_fast_assign(
-		strncpy_pad(__entry->name, dev_name(wb->bdi->dev), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
 		__entry->older	= dirtied_before;
 		__entry->age	= (jiffies - dirtied_before) * 1000 / HZ;
 		__entry->moved	= moved;
@@ -458,7 +455,7 @@ TRACE_EVENT(bdi_dirty_ratelimit,
 	),
 
 	TP_fast_assign(
-		strscpy_pad(__entry->bdi, dev_name(wb->bdi->dev), 32);
+		strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
 		__entry->write_bw	= KBps(wb->write_bandwidth);
 		__entry->avg_write_bw	= KBps(wb->avg_write_bandwidth);
 		__entry->dirty_rate	= KBps(dirty_rate);
@@ -523,7 +520,7 @@ TRACE_EVENT(balance_dirty_pages,
 
 	TP_fast_assign(
 		unsigned long freerun = (thresh + bg_thresh) / 2;
-		strscpy_pad(__entry->bdi, dev_name(wb->bdi->dev), 32);
+		strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
 
 		__entry->limit		= global_wb_domain.dirty_limit;
 		__entry->setpoint	= (global_wb_domain.dirty_limit +
@@ -584,7 +581,7 @@ TRACE_EVENT(writeback_sb_inodes_requeue,
 
 	TP_fast_assign(
 		strscpy_pad(__entry->name,
-			    dev_name(inode_to_bdi(inode)->dev), 32);
+			    bdi_dev_name(inode_to_bdi(inode)), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->state		= inode->i_state;
 		__entry->dirtied_when	= inode->dirtied_when;
@@ -658,7 +655,7 @@ DECLARE_EVENT_CLASS(writeback_single_inode_template,
 
 	TP_fast_assign(
 		strscpy_pad(__entry->name,
-			    dev_name(inode_to_bdi(inode)->dev), 32);
+			    bdi_dev_name(inode_to_bdi(inode)), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->state		= inode->i_state;
 		__entry->dirtied_when	= inode->dirtied_when;
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 72e6d0c55cfad..0a970be24a28d 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -19,6 +19,7 @@ struct backing_dev_info noop_backing_dev_info = {
 EXPORT_SYMBOL_GPL(noop_backing_dev_info);
 
 static struct class *bdi_class;
+const char *bdi_unknown_name = "(unknown)";
 
 /*
  * bdi_lock protects updates to bdi_list. bdi_list has RCU reader side
-- 
2.27.0




  parent reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-11 15:02 [PATCH 4.19 00/24] 4.19.176-rc1 review Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 01/24] tracing/kprobe: Fix to support kretprobe events on unloaded modules Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 02/24] block: fix NULL pointer dereference in register_disk Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 03/24] fgraph: Initialize tracing_graph_pause at task creation Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 04/24] remoteproc: qcom_q6v5_mss: Validate modem blob firmware size before load Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 05/24] remoteproc: qcom_q6v5_mss: Validate MBA " Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 06/24] af_key: relax availability checks for skb size calculation Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 07/24] regulator: core: avoid regulator_resolve_supply() race condition Greg Kroah-Hartman
2021-02-11 15:26   ` Mark Brown
2021-02-11 15:38     ` Greg Kroah-Hartman
2021-02-11 15:40       ` Mark Brown
2021-02-11 18:06         ` Sasha Levin
2021-02-11 15:02 ` [PATCH 4.19 08/24] chtls: Fix potential resource leak Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 09/24] pNFS/NFSv4: Try to return invalid layout in pnfs_layout_process() Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 10/24] iwlwifi: mvm: take mutex for calling iwl_mvm_get_sync_time() Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 11/24] iwlwifi: pcie: add a NULL check in iwl_pcie_txq_unmap Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 12/24] iwlwifi: pcie: fix context info memory leak Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 13/24] iwlwifi: mvm: guard against device removal in reprobe Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 14/24] SUNRPC: Move simple_get_bytes and simple_get_netobj into private header Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 15/24] SUNRPC: Handle 0 length opaque XDR object data properly Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 16/24] lib/string: Add strscpy_pad() function Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 17/24] include/trace/events/writeback.h: fix -Wstringop-truncation warnings Greg Kroah-Hartman
2021-02-11 15:02 ` Greg Kroah-Hartman [this message]
2021-02-11 15:02 ` [PATCH 4.19 19/24] Fix unsynchronized access to sev members through svm_register_enc_region Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 20/24] block: dont hold q->sysfs_lock in elevator_init_mq Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 21/24] blk-mq: dont hold q->sysfs_lock in blk_mq_map_swqueue Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 22/24] squashfs: add more sanity checks in id lookup Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 23/24] squashfs: add more sanity checks in inode lookup Greg Kroah-Hartman
2021-02-11 15:02 ` [PATCH 4.19 24/24] squashfs: add more sanity checks in xattr id lookup Greg Kroah-Hartman
2021-02-12  4:46 ` [PATCH 4.19 00/24] 4.19.176-rc1 review Naresh Kamboju
2021-02-12  7:42   ` Greg Kroah-Hartman
2021-02-12 10:29     ` Naresh Kamboju
2021-02-12 11:00       ` Greg Kroah-Hartman
2021-02-12 16:18 ` Shuah Khan

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