ecryptfs_lookup_interpose(): lower_dentry->d_parent is not stable either

BugLink: https://bugs.launchpad.net/bugs/1853881

commit 762c6968 upstream.

We need to get the underlying dentry of parent; sure, absent the races
it is the parent of underlying dentry, but there's nothing to prevent
losing a timeslice to preemtion in the middle of evaluation of
lower_dentry->d_parent->d_inode, having another process move lower_dentry
around and have its (ex)parent not pinned anymore and freed on memory
pressure.  Then we regain CPU and try to fetch ->d_inode from memory
that is freed by that point.

dentry->d_parent *is* stable here - it's an argument of ->lookup() and
we are guaranteed that it won't be moved anywhere until we feed it
to d_add/d_splice_alias.  So we safely go that way to get to its
underlying dentry.

Cc: stable@vger.kernel.org # since 2009 or so
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/ecryptfs/inode.c

@@ -330,9 +330,9 @@ static int ecryptfs_lookup_interpose(struct dentry *dentry,
 				     struct dentry *lower_dentry,
 				     struct inode *dir_inode)
 {
+	struct path *path = ecryptfs_dentry_to_lower_path(dentry->d_parent);
 	struct inode *inode, *lower_inode;
 	struct ecryptfs_dentry_info *dentry_info;
-	struct vfsmount *lower_mnt;
 	int rc = 0;
 
 	dentry_info = kmem_cache_alloc(ecryptfs_dentry_info_cache, GFP_KERNEL);
@@ -344,12 +344,11 @@ static int ecryptfs_lookup_interpose(struct dentry *dentry,
 		return -ENOMEM;
 	}
 
-	lower_mnt = mntget(ecryptfs_dentry_to_lower_mnt(dentry->d_parent));
-	fsstack_copy_attr_atime(dir_inode, d_inode(lower_dentry->d_parent));
+	fsstack_copy_attr_atime(dir_inode, d_inode(path->dentry));
 	BUG_ON(!d_count(lower_dentry));
 
 	ecryptfs_set_dentry_private(dentry, dentry_info);
-	dentry_info->lower_path.mnt = lower_mnt;
+	dentry_info->lower_path.mnt = mntget(path->mnt);
 	dentry_info->lower_path.dentry = lower_dentry;
 
 	/*