inode_permission NULL pointer dereference in 3.13-rc1

[Al Viro]

On Sun, Nov 24, 2013 at 06:04:13AM -0800, Christoph Hellwig wrote:
> Seems I can reproduce this by doing a full xfstests run and then
> shutting down the VM.  Doesn't seem to happen with the XFS tree
> which is still based on 3.12-rc1.

may_lookup() with LOOKUP_RCU in nd->flags and NULL nd->inode, by
the look of it...  Interesting.

AFAICS, path_init() initializes nd->inode on all success exits.
As for the places where we reassign nd->inode, we have the following:
fs/namei.c:681: nd->inode = nd->path.dentry->d_inode;
	nd_jump_link(), we would've already left RCU mode
fs/namei.c:856:         nd->inode = nd->path.dentry->d_inode;
	follow_link() - ditto.
fs/namei.c:1160:        nd->inode = nd->path.dentry->d_inode;
	follow_dotdot_rcu()
fs/namei.c:1257:        nd->inode = nd->path.dentry->d_inode;
	follow_dotdot() - not in RCU mode
fs/namei.c:1547:        nd->inode = inode;
	walk_component() - inode can't be NULL here, variable has
been explicitly compared with NULL a few lines prior
fs/namei.c:1825:                nd->inode = inode;
	path_init() with LOOKUP_ROOT; not from stat(2) and it would've
already oopsed with NULL inode anyway.
fs/namei.c:1892:        nd->inode = nd->path.dentry->d_inode;
	path_init()
fs/namei.c:3006:        nd->inode = inode;
	do_last(), not from stat(2) and no may_lookup() done past that
point anyway.
fs/namei.c:3077:        nd->inode = dir->d_inode;
	do_last(), not from stat(2)

which seems to leave two candidates - follow_dotdot_rcu() and path_init(),
both setting nd->inode to nd->path.dentry->d_inode...

Could you try to reproduce it with something like
	if (read_seqretry(&mount_lock, nd->m_seq))
		goto failed;
slapped before the success exit in follow_dotdot_rcu(), just to see if
we are hitting some races with umount here?