be careful with nd->inode in path_init() and follow_dotdot_rcu()

commit 4023bfc9 upstream.

in the former we simply check if dentry is still valid after picking
its ->d_inode; in the latter we fetch ->d_inode in the same places
where we fetch dentry and its ->d_seq, under the same checks.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/namei.c

@@ -1135,6 +1135,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 
 static int follow_dotdot_rcu(struct nameidata *nd)
 {
+	struct inode *inode = nd->inode;
 	if (!nd->root.mnt)
 		set_root_rcu(nd);
 
@@ -1148,6 +1149,7 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 			struct dentry *parent = old->d_parent;
 			unsigned seq;
 
+			inode = parent->d_inode;
 			seq = read_seqcount_begin(&parent->d_seq);
 			if (read_seqcount_retry(&old->d_seq, nd->seq))
 				goto failed;
@@ -1157,6 +1159,7 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 		}
 		if (!follow_up_rcu(&nd->path))
 			break;
+		inode = nd->path.dentry->d_inode;
 		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
 	}
 	while (d_mountpoint(nd->path.dentry)) {
@@ -1166,11 +1169,12 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 			break;
 		nd->path.mnt = &mounted->mnt;
 		nd->path.dentry = mounted->mnt.mnt_root;
+		inode = nd->path.dentry->d_inode;
 		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
 		if (!read_seqretry(&mount_lock, nd->m_seq))
 			goto failed;
 	}
-	nd->inode = nd->path.dentry->d_inode;
+	nd->inode = inode;
 	return 0;
 
 failed:
@@ -1899,7 +1903,14 @@ static int path_init(int dfd, const char *name, unsigned int flags,
 	}
 
 	nd->inode = nd->path.dentry->d_inode;
-	return 0;
+	if (!(flags & LOOKUP_RCU))
+		return 0;
+	if (likely(!read_seqcount_retry(&nd->path.dentry->d_seq, nd->seq)))
+		return 0;
+	if (!(nd->flags & LOOKUP_ROOT))
+		nd->root.mnt = NULL;
+	rcu_read_unlock();
+	return -ECHILD;
 }
 
 static inline int lookup_last(struct nameidata *nd, struct path *path)