Commit 1e9c75fb authored Oct 03, 2018 by Benjamin Coddington Committed by Eric W. Biederman Nov 12, 2018

mnt: fix __detach_mounts infinite loop



Since commit ff17fa56 ("d_invalidate(): unhash immediately")
immediately unhashes the dentry, we'll never return the mountpoint in
lookup_mountpoint(), which can lead to an unbreakable loop in
d_invalidate().

I have reports of NFS clients getting into this condition after the server
removes an export of an existing mount created through follow_automount(),
but I suspect there are various other ways to produce this problem if we
hunt down users of d_invalidate().  For example, it is possible to get into
this state by using XFS' d_invalidate() call in xfs_vn_unlink():

truncate -s 100m img{1,2}

mkfs.xfs -q -n version=ci img1
mkfs.xfs -q -n version=ci img2

mkdir -p /mnt/xfs
mount img1 /mnt/xfs

mkdir /mnt/xfs/sub1
mount img2 /mnt/xfs/sub1

cat > /mnt/xfs/sub1/foo &
umount -l /mnt/xfs/sub1
mount img2 /mnt/xfs/sub1

mount --make-private /mnt/xfs

mkdir /mnt/xfs/sub2
mount --move /mnt/xfs/sub1 /mnt/xfs/sub2
rmdir /mnt/xfs/sub1

Fix this by moving the check for an unlinked dentry out of the
detach_mounts() path.

Fixes: ff17fa56 ("d_invalidate(): unhash immediately")
Cc: stable@vger.kernel.org
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

parent 9c8e0a1b

fs/namespace.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -695,9 +695,6 @@ static struct mountpoint lookup_mountpoint(struct dentry dentry)

		hlist_for_each_entry(mp, chain, m_hash) {
		if (mp->m_dentry == dentry) {
		/* might be worth a WARN_ON() */
		if (d_unlinked(dentry))
		return ERR_PTR(-ENOENT);
		mp->m_count++;
		return mp;
		}
		@@ -711,6 +708,9 @@ static struct mountpoint get_mountpoint(struct dentry dentry)
		int ret;

		if (d_mountpoint(dentry)) {
		/* might be worth a WARN_ON() */
		if (d_unlinked(dentry))
		return ERR_PTR(-ENOENT);
		mountpoint:
		read_seqlock_excl(&mount_lock);
		mp = lookup_mountpoint(dentry);

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