Make sure nd->path.mnt and nd->path.dentry are always valid pointers

Initialize them in set_nameidata() and make sure that terminate_walk() clears them
once the pointers become potentially invalid (i.e. we leave RCU mode or drop them
in non-RCU one).  Currently we have "path_init() always initializes them and nobody
accesses them outside of path_init()/terminate_walk() segments", which is asking
for trouble.

With that change we would have nd->path.{mnt,dentry}
	1) always valid - NULL or pointing to currently allocated objects.
	2) non-NULL while we are successfully walking
	3) NULL when we are not walking at all
	4) contributing to refcounts whenever non-NULL outside of RCU mode.

Fixes: 6c6ec2b0a3e0 ("fs: add support for LOOKUP_CACHED")
Reported-by: syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com
Tested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 216f16e..fc8760d 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -579,6 +579,8 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
        p->stack = p->internal;
        p->dfd = dfd;
        p->name = name;
+       p->path.mnt = NULL;
+       p->path.dentry = NULL;
        p->total_link_count = old ? old->total_link_count : 0;
        p->saved = old;
        current->nameidata = p;
@@ -652,6 +654,8 @@ static void terminate_walk(struct nameidata *nd)
                rcu_read_unlock();
        }
        nd->depth = 0;
+       nd->path.mnt = NULL;
+       nd->path.dentry = NULL;
 }
 
 /* path_put is needed afterwards regardless of success or failure */
@@ -2322,8 +2326,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
        }
 
        nd->root.mnt = NULL;
-       nd->path.mnt = NULL;
-       nd->path.dentry = NULL;
 
        /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
        if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {