Re: [PATCH] nfsd: check for oversized NFSv2/v3 arguments

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Apr 24, 2017 at 08:21:36AM +1000, NeilBrown wrote:
> On Fri, Apr 21 2017, J. Bruce Fields wrote:
> 
> > On Thu, Apr 20, 2017 at 12:19:35PM -0400, J. Bruce Fields wrote:
> >> On Tue, Apr 18, 2017 at 01:13:51PM -0400, J. Bruce Fields wrote:
> >> > On Tue, Apr 18, 2017 at 10:25:20AM +1000, NeilBrown wrote:
> >> > >  I can't say that I like this patch at all.
> >> > > 
> >> > > The problem is that:
> >> > > 
> >> > > 	pages = size / PAGE_SIZE + 1; /* extra page as we hold both request and reply.
> >> > > 				       * We assume one is at most one page
> >> > > 				       */
> >> > > 
> >> > > this assumption is never verified.
> >> > > To my mind, the "obvious" way to verify this assumption is that an
> >> > > attempt to generate a multi-page reply should fail if there was a
> >> > > multi-page request.
> >> > 
> >> > A third option, by the way, which Ari Kauppi argued for, is adding a
> >> > null check each time we increment rq_next_page, since we seem to arrange
> >> > for the page array to always be NULL-terminated.
> >> > 
> >> > > Failing if there was a little bit of extra noise at the end of the
> >> > > request seems harsher than necessary, and could result in a regression.
> >> > 
> >> > You're worrying there might be a weird old client out there somewhere?
> >> > I guess it seems like a small enough risk to me.  I'm more worried the
> >> > extra garbage might violate assumptions elsewhere in the code.
> >> > 
> >> > But, this looks good too:
> >> 
> >> But, I'm not too happy about putting that NFSv2/v3-specific check in
> >> common rpc code.
> >
> > Well, but it should work just as well in nfsd_dispatch, I think?
> > (Untested).  So, maybe that's simplest as a first step:
> >
> > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > index 31e1f9593457..b6298d30a01f 100644
> > --- a/fs/nfsd/nfssvc.c
> > +++ b/fs/nfsd/nfssvc.c
> > @@ -759,6 +759,22 @@ nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
> >  				rqstp->rq_vers, rqstp->rq_proc);
> >  	proc = rqstp->rq_procinfo;
> >  
> > +	if (rqstp->rq_vers < 4 &&
> > +	    (proc->pc_xdrressize == 0
> > +			|| proc->pc_xdrressize > XDR_QUADLEN(PAGE_SIZE))
> > +		&& rqstp->rq_arg.len > PAGE_SIZE) {
> > +		/*
> > +		 * NFSv2 and v3 assume that an operation may have either a
> > +		 * large argument, or a large reply, but never both.
> > +		 *
> > +		 * NFSv4 may handle compounds with both argument and
> > +		 * reply larger than a reply; it has more xdr careful
> > +		 * xdr decoding which can handle such calls safely.
> > +		 */
> > +		dprintk("nfsd: NFSv%d argument too large\n", rqstp->rq_vers);
> > +		*statp = rpc_garbage_args;
> > +		return 1;
> > +	}
> >  	/*
> >  	 * Give the xdr decoder a chance to change this if it wants
> >  	 * (necessary in the NFSv4.0 compound case)
> 
> I like this.  I think this should be the basis of what goes to -stable,
> and other improvements should stay in mainline.
> 
> The only change I would suggest would be to be explicit about where the
> nfsacl protocol fits with this.

Oh, good point, I'd forgotten nfsd_dispatch handles multiple protocols!

> We could change "rqstp->rq_vers < 4" to
>  "rqstp->rq_prog == NFS_PROGRAM && rqstp->rq_vers < 4"
> or we could change the text:
>  NFSv2 and v3 assume ...
> to
>  NFSv2 and v3, along with NFSASL, assume ...
> 
> and possibly change "rqstp->rq_vers < 4" to "!nfsd_v4client(rqstp)".
> 
> I believe none of this applies to lockd as none of that code ever looks
> beyond a single page.

That makes sense.

--b.