vfs: fix race in rcu lookup of pruned dentry

Don't update *inode in __follow_mount_rcu() until we'd verified that
there is mountpoint there.  Kudos to Hugh Dickins for catching that
one in the first place and eventually figuring out the solution (and
catching a braino in the earlier version of patch).
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -942,7 +942,6 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		*inode = path->dentry->d_inode;
 		if (unlikely(managed_dentry_might_block(path->dentry)))
 			return false;
 
@@ -955,6 +954,12 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		path->mnt = mounted;
 		path->dentry = mounted->mnt_root;
 		nd->seq = read_seqcount_begin(&path->dentry->d_seq);
+		/*
+		 * Update the inode too. We don't need to re-check the
+		 * dentry sequence number here after this d_inode read,
+		 * because a mount-point is always pinned.
+		 */
+		*inode = path->dentry->d_inode;
 	}
 	return true;
 }