5.4-stable review patch. If anyone has any objections, please let me know.
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From: Al Viro viro@zeniv.linux.org.uk
[ Upstream commit 65781e19dcfcb4aed1167d87a3ffcc2a0c071d47 ]
do_umount() analogue of the race fixed in 119e1ef80ecf "fix __legitimize_mnt()/mntput() race". Here we want to make sure that if __legitimize_mnt() doesn't notice our lock_mount_hash(), we will notice their refcount increment. Harder to hit than mntput_no_expire() one, fortunately, and consequences are milder (sync umount acting like umount -l on a rare race with RCU pathwalk hitting at just the wrong time instead of use-after-free galore mntput_no_expire() counterpart used to be hit). Still a bug...
Fixes: 48a066e72d97 ("RCU'd vfsmounts") Reviewed-by: Christian Brauner brauner@kernel.org Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Sasha Levin sashal@kernel.org --- fs/namespace.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c index 281f08eaba5b9..809ec3acb147c 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -569,7 +569,7 @@ int __legitimize_mnt(struct vfsmount *bastard, unsigned seq) return 0; mnt = real_mount(bastard); mnt_add_count(mnt, 1); - smp_mb(); // see mntput_no_expire() + smp_mb(); // see mntput_no_expire() and do_umount() if (likely(!read_seqretry(&mount_lock, seq))) return 0; if (bastard->mnt_flags & MNT_SYNC_UMOUNT) { @@ -1588,6 +1588,7 @@ static int do_umount(struct mount *mnt, int flags) umount_tree(mnt, UMOUNT_PROPAGATE); retval = 0; } else { + smp_mb(); // paired with __legitimize_mnt() shrink_submounts(mnt); retval = -EBUSY; if (!propagate_mount_busy(mnt, 2)) {