namei: ->d_inode of a pinned dentry is stable only for positives

[ Upstream commit d4565649 ]

both do_last() and walk_component() risk picking a NULL inode out
of dentry about to become positive, *then* checking its flags and
seeing that it's not negative anymore and using (already stale by
then) value they'd fetched earlier.  Usually ends up oopsing soon
after that...

Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

fs/namei.c

@@ -1619,10 +1619,10 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
 		if (err < 0)
 			goto out_err;
 
-		inode = path->dentry->d_inode;
 		err = -ENOENT;
 		if (d_is_negative(path->dentry))
 			goto out_path_put;
+		inode = path->dentry->d_inode;
 	}
 
 	if (should_follow_link(path->dentry, follow)) {
@@ -3078,6 +3078,7 @@ static int do_last(struct nameidata *nd, struct path *path,
 		path_to_nameidata(path, nd);
 		goto out;
 	}
+	inode = path->dentry->d_inode;
 finish_lookup:
 	/* we _can_ be in RCU mode here */
 	if (should_follow_link(path->dentry, !symlink_ok)) {