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")
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/namespace.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index eba4748388b1..d8a344d0a80a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -787,7 +787,7 @@ 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;
lock_mount_hash();
@@ -2044,6 +2044,7 @@ 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)) {
--
2.39.5
