Re: fs: use-after-free in path_lookupat

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sun, Mar 05, 2017 at 12:37:13PM +0100, Dmitry Vyukov wrote:

> I am pretty sure it is that one.
> I don't think I ever used name_to_handle_at syscall in my life and I
> definitely didn't make it lookup a memfd :)

So what does it normally return?  On the runs where we do not hit that
use-after-free, that is.

What gets triggered there is nd->path.dentry pointing to already freed
dentry.  We are in RCU mode, so we are not pinning the dentry and it
might have reached dentry_free().  However, anything with DCACHE_RCUACCESS
set would have freeing RCU-delayed, making that impossible.

memfd stuff does *not* have DCACHE_RCUACCESS, which would've made it
plausible, but... there we really should've been stopped cold by
the d_can_lookup() check - that is done while we are still holding
a reference to struct file, which should've prevented freeing and
reuse.  So at the time of that check we have dentry still not reused
by anything, and d_can_lookup() should've failed.

There is a race that could bugger the things up in that area, but it needs
empty name, so this one is something else...