[PATCH 6.1.y] smb: smbdirect: fix buffer length underflow in RDMA transfers

[Sasha Levin <sashal@kernel.org>]

From: Stefan Metzmacher <metze@samba.org>

[ Upstream commit 7193241a49c958782ba4b8df06da3734d1e317d2 ]

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: e628bf939aaf ("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>
[ sashal: adjusted context for smb_direct_rdma_xmit() using calc_rw_credits() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/smb/server/transport_rdma.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/smb/server/transport_rdma.c b/fs/smb/server/transport_rdma.c
index bcf0631ce10a6..cc9409bb5404c 100644
--- a/fs/smb/server/transport_rdma.c
+++ b/fs/smb/server/transport_rdma.c
@@ -1378,7 +1378,6 @@ static int smb_direct_rdma_xmit(struct smb_direct_transport *t,
 		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 += calc_rw_credits(t, desc_buf, desc_buf_len);
-- 
2.53.0