NFSv4: Ensure decode_compound_hdr() sanity checks the tag

The server is supposed to return the same tag that the client sends in
the outgoing RPC call, but we should still sanity check the length just
in case.

Reported-by: <rtm@csail.mit.edu>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4xdr.c

@@ -3168,20 +3168,23 @@ static int decode_opaque_inline(struct xdr_stream *xdr, unsigned int *len, char
 
 static int decode_compound_hdr(struct xdr_stream *xdr, struct compound_hdr *hdr)
 {
-	__be32 *p;
+	ssize_t ret;
+	void *ptr;
+	u32 tmp;
 
-	p = xdr_inline_decode(xdr, 8);
-	if (unlikely(!p))
+	if (xdr_stream_decode_u32(xdr, &tmp) < 0)
 		return -EIO;
-	hdr->status = be32_to_cpup(p++);
-	hdr->taglen = be32_to_cpup(p);
+	hdr->status = tmp;
 
-	p = xdr_inline_decode(xdr, hdr->taglen + 4);
-	if (unlikely(!p))
+	ret = xdr_stream_decode_opaque_inline(xdr, &ptr, NFS4_OPAQUE_LIMIT);
+	if (ret < 0)
+		return -EIO;
+	hdr->taglen = ret;
+	hdr->tag = ptr;
+
+	if (xdr_stream_decode_u32(xdr, &tmp) < 0)
 		return -EIO;
-	hdr->tag = (char *)p;
-	p += XDR_QUADLEN(hdr->taglen);
-	hdr->nops = be32_to_cpup(p);
+	hdr->nops = tmp;
 	if (unlikely(hdr->nops < 1))
 		return nfs4_stat_to_errno(hdr->status);
 	return 0;