Re: NFS/credentials leak in 2.6.29-rc1

[Evgeniy Polyakov <zbr@ioremap.net>]

On Wed, Jan 21, 2009 at 12:23:09PM +0000, David Howells (dhowells@redhat.com) wrote:
> J. Bruce Fields <bfields@fieldses.org> wrote:
> 
> > 	- Finally, we put_cred(override_creds(new)).  That modifies
> > 	  current->cred again, putting the old value and getting the
> > 	  new.
> > 
> > Hm.  But that last part's not OK; aren't we still holding our own
> > reference to new, in addition to the one that override_creds() just
> > took?  So I think we need the following?
> 
> Yes, you're right.  override_creds() takes an extra ref on the argument it is
> passed, thus leaving the caller with their original reference intact.
> 
> So really, you don't want to call override_creds() as that will cost you an
> extra atomic_inc() and atomic_dec_and_test().  I recommend you replace:
> 
>         put_cred(override_creds(new));
> 
> with:
> 
> 	revert_creds(new);
> 
> I think that should do the right thing.  It may look a bit odd, but it'll be
> quicker.  If you object to using revert_creds)( because of the name, we can
> come up with an alternative name.

With additional put_cred, i.e.:

        put_cred(override_creds(new));
        put_cred(new);
        return 0;

I got following fun tcpdump and failed mount (it stuck, but can be interrupted):

15:34:41.253911 IP 77.88.20.183.1835336279 > 77.88.20.182.2049: 44 null
15:34:41.253916 IP 77.88.20.182.2049 > 77.88.20.183.728: . ack 44 win 88 <nop,nop,timestamp 125462 37402358>
15:34:41.254103 IP 77.88.20.182.2049 > 77.88.20.183.1835336279: reply ok 28 null
15:34:41.254229 IP 77.88.20.183.728 > 77.88.20.182.2049: . ack 29 win 89 <nop,nop,timestamp 37402358 125463>
15:34:41.254238 IP 77.88.20.183.1852113495 > 77.88.20.182.2049: 44 null
15:34:41.254271 IP 77.88.20.182.2049 > 77.88.20.183.1852113495: reply ok 28 null
15:34:41.254378 IP 77.88.20.183.1868890711 > 77.88.20.182.2049: 100 fsinfo [|nfs]
15:34:41.254411 IP 77.88.20.182.2049 > 77.88.20.183.1868890711: reply ok 36 fsinfo [|nfs]
15:34:41.254528 IP 77.88.20.183.1885667927 > 77.88.20.182.2049: 100 fsinfo [|nfs]
15:34:41.254555 IP 77.88.20.182.2049 > 77.88.20.183.1885667927: reply ok 36 fsinfo [|nfs]

But no corruption in the dmesg (like oops or bug).

-- 
	Evgeniy Polyakov