NFSD: Prevent a potential integer overflow

commit 7f33b92e5b18e904a481e6e208486da43e4dc841 upstream.

If the tag length is >= U32_MAX - 3 then the "length + 4" addition
can result in an integer overflow. Address this by splitting the
decoding into several steps so that decode_cb_compound4res() does
not have to perform arithmetic on the unsafe length value.

Reported-by: Dan Carpenter <[email protected]>
Cc: [email protected]
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/nfsd/nfs4callback.c

@@ -283,17 +283,17 @@ static int decode_cb_compound4res(struct xdr_stream *xdr,
 	u32 length;
 	__be32 *p;
 
-	p = xdr_inline_decode(xdr, 4 + 4);
+	p = xdr_inline_decode(xdr, XDR_UNIT);
 	if (unlikely(p == NULL))
 		goto out_overflow;
-	hdr->status = be32_to_cpup(p++);
+	hdr->status = be32_to_cpup(p);
 	/* Ignore the tag */
-	length = be32_to_cpup(p++);
-	p = xdr_inline_decode(xdr, length + 4);
-	if (unlikely(p == NULL))
+	if (xdr_stream_decode_u32(xdr, &length) < 0)
+		goto out_overflow;
+	if (xdr_inline_decode(xdr, length) == NULL)
+		goto out_overflow;
+	if (xdr_stream_decode_u32(xdr, &hdr->nops) < 0)
 		goto out_overflow;
-	p += XDR_QUADLEN(length);
-	hdr->nops = be32_to_cpup(p);
 	return 0;
 out_overflow:
 	return -EIO;