smb: smbdirect: fix buffer length underflow in RDMA transfers

When a remote buffer descriptor is larger than the remaining local
buffer, smbdirect_connection_rdma_xmit() clamps its length and sets
buf_len to zero. The common accounting code then subtracts the clamped
length from buf_len, causing it to underflow. If another descriptor
follows, the loop processes it with a wrapped remaining length and
includes memory beyond the local buffer in the RDMA transfer.

A client can trigger an out-of-bounds kernel write with an SMB WRITE
request, or an out-of-bounds kernel read with an SMB READ request, by
supplying multiple RDMA buffer descriptors with an oversized descriptor
before the last one.

Remove the premature assignment to buf_len. Subtracting the clamped length
already consumes the remaining buffer, so the next iteration stops and
only descriptors within the local buffer are used to build work requests.

Fixes: e628bf939a ("ksmbd: reduce descriptor size if remaining bytes is less than request size")
Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-33549
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/smb/smbdirect/rw.c

@@ -143,7 +143,6 @@ int smbdirect_connection_rdma_xmit(struct smbdirect_socket *sc,
 		if (desc_buf_len > buf_len) {
 			desc_buf_len = buf_len;
 			desc[i].length = cpu_to_le32(desc_buf_len);
-			buf_len = 0;
 		}
 
 		credits_needed += smbdirect_connection_calc_rw_credits(sc,