From 187d844f2e98f432f0c19c18b46afc233d094168 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Fri, 29 Sep 2023 20:24:34 -0400 Subject: [PATCH] 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 Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- 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 b165377179c3..6774e1fcf7c5 100644 --- a/fs/afs/file.c +++ b/fs/afs/file.c @@ -512,13 +512,17 @@ 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);