Re: [PATCH v2] mnt: fix __detach_mounts infinite loop

["Benjamin Coddington" <bcodding@redhat.com>]

Al, would you be willing to take a look at this one?

Ben

On 3 Oct 2018, at 10:18, Benjamin Coddington wrote:

> v2: less whitespace
>
> 8<------------------------------------------------------------------------
>
> Since commit ff17fa561a04 ("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: ff17fa561a04 ("d_invalidate(): unhash immediately")
> Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> ---
>  fs/namespace.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index bd2f4c68506a..ad8feb9371f2 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -780,9 +780,6 @@ 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;
>  		}
> @@ -796,6 +793,9 @@ 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);
> -- 
> 2.14.3