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afs: fix __afs_break_callback() / afs_drop_open_mmap() race 

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@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2023-09-29 20:24:34 -04:00
parent af072cf683
commit 275655d320
1 changed files with 6 additions and 2 deletions
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8
fs/afs/file.c
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@ -417,13 +417,17 @@ 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->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->open_mmaps_lock);
flush_work(&vnode->cb_work);