Re: fs: use-after-free in path_lookupat

[Dmitry Vyukov]

On Sun, Mar 5, 2017 at 4:57 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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...

You can see from the log above that s always empty somehow, so the
d_can_lookup check is simply not done. I have not looked at the code,
but it's not racy, so should follow from the arguments passed to
name_to_handle_at.