Re: Remote Linux Kernel < 2.4.21 DoS in XDR routine.

[Jared Stanbrough <jareds () pdx edu>]

Steve,

The attached code requires that an export has been previously mounted onto
the attacker's machine. I haven't done much testing to determine whether
or not it is vulnerable before authentication takes place. Since the
procedures which operate on file handles are only called after a client is
authenticated, I believe that only people in the exports can trigger it.
If anyone can trigger it otherwise I'd like to hear, though...

--jared

On Tue, 29 Jul 2003, Stephen Clowater wrote:

> What does one need for this attack to work?
>
> Can it only be launched by people in the exports ? Or can it be launched by
> anyone?
>
> Thanks
>
> Steve
> ----- Original Message -----
> From: "Jared Stanbrough" <jareds () pdx edu>
> To: <bugtraq () securityfocus com>
> Sent: Tuesday, July 29, 2003 4:55 PM
> Subject: Remote Linux Kernel < 2.4.21 DoS in XDR routine.
>
>
> > Hello all,
> >
> > I have discovered a signed/unsigned issue in a routine responsible for
> > demarshalling XDR data for NFSv3 procedure calls. As far as I can tell,
> > this bug has existed since NFSv3 support was integrated. It has been
> > silently fixed in 2.4.21.
> >
> > The bug is in the decode_fh routine of fs/nfsd/nfs3xdr.c under the kernel
> > source tree.
> >
> > Vulnerable code:
> >
> > static inline u32 *
> > decode_fh(u32 *p, struct svc_fh *fhp)
> > {
> >         int size;
> >         fh_init(fhp, NFS3_FHSIZE);
> >         size = ntohl(*p++);
> >         if (size > NFS3_FHSIZE)
> >                 return NULL;
> >
> >         memcpy(&fhp->fh_handle.fh_base, p, size);
> >         fhp->fh_handle.fh_size = size;
> >         return p + XDR_QUADLEN(size);
> > }
> >
> > Where p is a packet of attacker controlled XDR data. If size is made to be
> > negative, the sanity check is passed and the malicious value is passed to
> > memcpy. Due to the behavior of the kernel's memcpy, this will cause a very
> > large copy in kernel space, resulting in an instant kernel panic.
> >
> > The attached code is a POC of this vulnerability. It requires that the
> > vulnerable host has an exported directory available to the attacker. This
> > is probably not the only way to manifest this bug, however.
> >
> > If you have any questions, please feel free to contact me.
> >
> > Cheers,
> >
> > Jared Stanbrough <jareds () pdx edu>