linux-ext4 - [PATCH 06/13] afs: fix __afs_break_callback() / afs_drop_open

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Date: Sun,  4 Feb 2024 02:17:32 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: linux-fsdevel@...r.kernel.org
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	linux-ext4@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-cifs@...r.kernel.org
Subject: [PATCH 06/13] 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).

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/afs/file.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/afs/file.c b/fs/afs/file.c
index 3d33b221d9ca..ef2cc8f565d2 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -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);
-- 
2.39.2