Commit 65781e19 authored by Al Viro's avatar Al Viro
Browse files

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



do_umount() analogue of the race fixed in 119e1ef8 "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: 48a066e7 ("RCU'd vfsmounts")
Reviewed-by: default avatarChristian Brauner <brauner@kernel.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 250cf369

fs/namespace.c

+2 −1
Original line number Diff line number Diff line
@@ -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)) {