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do_umount(): add missing barrier before refcount checks in sync case

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[ Upstream commit 65781e19dcfcb4aed1167d87a3ffcc2a0c071d47 ]

do_umount() analogue of the race fixed in 119e1ef80e "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: 48a066e72d ("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>
This commit is contained in:
Al Viro 2025-04-28 23:56:14 -04:00 • committed by Greg Kroah-Hartman
commit 80a00ec057
1 changed files with 2 additions and 1 deletions
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3
fs/namespace.c
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@ -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)) {