NFS: Fix listxattr receive buffer size

Certain NFSv4.2/RDMA tests fail with v5.9-rc1.

rpcrdma_convert_kvec() runs off the end of the rl_segments array
because rq_rcv_buf.tail[0].iov_len holds a very large positive
value. The resultant kernel memory corruption is enough to crash
the client system.

Callers of rpc_prepare_reply_pages() must reserve an extra XDR_UNIT
in the maximum decode size for a possible XDR pad of the contents
of the xdr_buf's pages. That guarantees the allocated receive buffer
will be large enough to accommodate the usual contents plus that XDR
pad word.

encode_op_hdr() cannot add that extra word. If it does,
xdr_inline_pages() underruns the length of the tail iovec.

Fixes: 3e1f0212 ("NFSv4.2: add client side XDR handling for extended attributes")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs42xdr.c

@@ -196,7 +196,7 @@
 				 1 + nfs4_xattr_name_maxsz + 1)
 #define decode_setxattr_maxsz   (op_decode_hdr_maxsz + decode_change_info_maxsz)
 #define encode_listxattrs_maxsz  (op_encode_hdr_maxsz + 2 + 1)
-#define decode_listxattrs_maxsz  (op_decode_hdr_maxsz + 2 + 1 + 1)
+#define decode_listxattrs_maxsz  (op_decode_hdr_maxsz + 2 + 1 + 1 + 1)
 #define encode_removexattr_maxsz (op_encode_hdr_maxsz + 1 + \
 				  nfs4_xattr_name_maxsz)
 #define decode_removexattr_maxsz (op_decode_hdr_maxsz + \
@@ -531,7 +531,7 @@ static void encode_listxattrs(struct xdr_stream *xdr,
 {
 	__be32 *p;
 
-	encode_op_hdr(xdr, OP_LISTXATTRS, decode_listxattrs_maxsz + 1, hdr);
+	encode_op_hdr(xdr, OP_LISTXATTRS, decode_listxattrs_maxsz, hdr);
 
 	p = reserve_space(xdr, 12);
 	if (unlikely(!p))