svcrdma: Address an integer overflow

svcrdma: Address an integer overflow

Dan Carpenter reports:
> Commit 78147ca8b4a9 ("svcrdma: Add a "parsed chunk list" data
> structure") from Jun 22, 2020 (linux-next), leads to the following
> Smatch static checker warning:
>
>	net/sunrpc/xprtrdma/svc_rdma_recvfrom.c:498 xdr_check_write_chunk()
>	warn: potential user controlled sizeof overflow 'segcount * 4 * 4'
>
> net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>     488 static bool xdr_check_write_chunk(struct svc_rdma_recv_ctxt *rctxt)
>     489 {
>     490         u32 segcount;
>     491         __be32 *p;
>     492
>     493         if (xdr_stream_decode_u32(&rctxt->rc_stream, &segcount))
>                                                               ^^^^^^^^
>
>     494                 return false;
>     495
>     496         /* A bogus segcount causes this buffer overflow check to fail. */
>     497         p = xdr_inline_decode(&rctxt->rc_stream,
> --> 498                               segcount * rpcrdma_segment_maxsz * sizeof(*p));
>
>
> segcount is an untrusted u32.  On 32bit systems anything >= SIZE_MAX / 16 will
> have an integer overflow and some those values will be accepted by
> xdr_inline_decode().

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 78147ca8b4a9 ("svcrdma: Add a "parsed chunk list" data structure")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>