ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable

lower_dentry can't go from positive to negative (we have it pinned),
but it *can* go from negative to positive.  So fetching ->d_inode
into a local variable, doing a blocking allocation, checking that
now ->d_inode is non-NULL and feeding the value we'd fetched
earlier to a function that won't accept NULL is not a good idea.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ecryptfs/inode.c

@@ -319,7 +319,7 @@ static int ecryptfs_i_size_read(struct dentry *dentry, struct inode *inode)
 static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,
 				     struct dentry *lower_dentry)
 {
-	struct inode *inode, *lower_inode = d_inode(lower_dentry);
+	struct inode *inode, *lower_inode;
 	struct ecryptfs_dentry_info *dentry_info;
 	struct vfsmount *lower_mnt;
 	int rc = 0;
@@ -339,7 +339,15 @@ static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,
 	dentry_info->lower_path.mnt = lower_mnt;
 	dentry_info->lower_path.dentry = lower_dentry;
 
-	if (d_really_is_negative(lower_dentry)) {
+	/*
+	 * negative dentry can go positive under us here - its parent is not
+	 * locked.  That's OK and that could happen just as we return from
+	 * ecryptfs_lookup() anyway.  Just need to be careful and fetch
+	 * ->d_inode only once - it's not stable here.
+	 */
+	lower_inode = READ_ONCE(lower_dentry->d_inode);
+
+	if (!lower_inode) {
 		/* We want to add because we couldn't find in lower */
 		d_add(dentry, NULL);
 		return NULL;