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

Stefan Bader stefan.bader at canonical.com
Tue Jul 24 12:35:31 UTC 2018


On 24.07.2018 11:11, Paolo Pisati wrote:
> From: "J. Bruce Fields" <bfields at redhat.com>
> 
> CVE-2017-7645
> 
> A client can append random data to the end of an NFSv2 or NFSv3 RPC call
> without our complaining; we'll just stop parsing at the end of the
> expected data and ignore the rest.
> 
> Encoded arguments and replies are stored together in an array of pages,
> and if a call is too large it could leave inadequate space for the
> reply.  This is normally OK because NFS RPC's typically have either
> short arguments and long replies (like READ) or long arguments and short
> replies (like WRITE).  But a client that sends an incorrectly long reply
> can violate those assumptions.  This was observed to cause crashes.
> 
> Also, several operations increment rq_next_page in the decode routine
> before checking the argument size, which can leave rq_next_page pointing
> well past the end of the page array, causing trouble later in
> svc_free_pages.
> 
> So, following a suggestion from Neil Brown, add a central check to
> enforce our expectation that no NFSv2/v3 call has both a large call and
> a large reply.
> 
> As followup we may also want to rewrite the encoding routines to check
> more carefully that they aren't running off the end of the page array.
> 
> We may also consider rejecting calls that have any extra garbage
> appended.  That would be safer, and within our rights by spec, but given
> the age of our server and the NFS protocol, and the fact that we've
> never enforced this before, we may need to balance that against the
> possibility of breaking some oddball client.
> 
> Reported-by: Tuomas Haanpää <thaan at synopsys.com>
> Reported-by: Ari Kauppi <ari at synopsys.com>
> Cc: stable at vger.kernel.org
> Reviewed-by: NeilBrown <neilb at suse.com>
> Signed-off-by: J. Bruce Fields <bfields at redhat.com>
> (cherry picked from commit e6838a29ecb484c97e4efef9429643b9851fba6e)
> Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---
>  fs/nfsd/nfssvc.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index 4942f43..a090399 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -628,6 +628,37 @@ static __be32 map_new_errors(u32 vers, __be32 nfserr)
>  	return nfserr;
>  }
>  
> +/*
> + * A write procedure can have a large argument, and a read procedure can
> + * have a large reply, but no NFSv2 or NFSv3 procedure has argument and
> + * reply that can both be larger than a page.  The xdr code has taken
> + * advantage of this assumption to be a sloppy about bounds checking in
> + * some cases.  Pending a rewrite of the NFSv2/v3 xdr code to fix that
> + * problem, we enforce these assumptions here:
> + */
> +static bool nfs_request_too_big(struct svc_rqst *rqstp,
> +				struct svc_procedure *proc)
> +{
> +	/*
> +	 * The ACL code has more careful bounds-checking and is not
> +	 * susceptible to this problem:
> +	 */
> +	if (rqstp->rq_prog != NFS_PROGRAM)
> +		return false;
> +	/*
> +	 * Ditto NFSv4 (which can in theory have argument and reply both
> +	 * more than a page):
> +	 */
> +	if (rqstp->rq_vers >= 4)
> +		return false;
> +	/* The reply will be small, we're OK: */
> +	if (proc->pc_xdrressize > 0 &&
> +	    proc->pc_xdrressize < XDR_QUADLEN(PAGE_SIZE))
> +		return false;
> +
> +	return rqstp->rq_arg.len > PAGE_SIZE;
> +}
> +
>  int
>  nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
>  {
> @@ -640,6 +671,11 @@ nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
>  				rqstp->rq_vers, rqstp->rq_proc);
>  	proc = rqstp->rq_procinfo;
>  
> +	if (nfs_request_too_big(rqstp, proc)) {
> +		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)
> 


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