This is a note to let you know that I've just added the patch titled
afs: fix __afs_break_callback() / afs_drop_open_mmap() race
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
afs-fix-__afs_break_callback-afs_drop_open_mmap-race.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 7de1d889e41c98b30bb38fd49bb7b85cc3d4714b
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Fri Sep 29 20:24:34 2023 -0400
afs: fix __afs_break_callback() / afs_drop_open_mmap() race
[ Upstream commit 275655d3207b9e65d1561bf21c06a622d9ec1d43 ]
In __afs_break_callback() we might check ->cb_nr_mmap and if it's non-zero
do queue_work(&vnode->cb_work). In afs_drop_open_mmap() we decrement
->cb_nr_mmap and do flush_work(&vnode->cb_work) if it reaches zero.
The trouble is, there's nothing to prevent __afs_break_callback() from
seeing ->cb_nr_mmap before the decrement and do queue_work() after both
the decrement and flush_work(). If that happens, we might be in trouble -
vnode might get freed before the queued work runs.
__afs_break_callback() is always done under ->cb_lock, so let's make
sure that ->cb_nr_mmap can change from non-zero to zero while holding
->cb_lock (the spinlock component of it - it's a seqlock and we don't
need to mess with the counter).
Acked-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/fs/afs/file.c b/fs/afs/file.c
index d37dd201752ba..0012ea300eb53 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -529,13 +529,17 @@ static void afs_add_open_mmap(struct afs_vnode *vnode)
static void afs_drop_open_mmap(struct afs_vnode *vnode)
{
- if (!atomic_dec_and_test(&vnode->cb_nr_mmap))
+ if (atomic_add_unless(&vnode->cb_nr_mmap, -1, 1))
return;
down_write(&vnode->volume->cell->fs_open_mmaps_lock);
- if (atomic_read(&vnode->cb_nr_mmap) == 0)
+ read_seqlock_excl(&vnode->cb_lock);
+ // the only place where ->cb_nr_mmap may hit 0
+ // see __afs_break_callback() for the other side...
+ if (atomic_dec_and_test(&vnode->cb_nr_mmap))
list_del_init(&vnode->cb_mmap_link);
+ read_sequnlock_excl(&vnode->cb_lock);
up_write(&vnode->volume->cell->fs_open_mmaps_lock);
flush_work(&vnode->cb_work);
