FAILED: patch "[PATCH] smb: smbdirect: fix buffer length underflow in RDMA transfers" failed to apply to 6.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 7193241a49c958782ba4b8df06da3734d1e317d2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026110437-bully-enviable-15b3@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 7193241a49c958782ba4b8df06da3734d1e317d2 Mon Sep 17 00:00:00 2001
From: Stefan Metzmacher <metze@samba.org>
Date: Fri, 23 Oct 2026 19:26:09 +0900
Subject: [PATCH] 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: 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>

diff --git a/fs/smb/smbdirect/rw.c b/fs/smb/smbdirect/rw.c
index 6fe38042cfb9..cb142c3c50b2 100644
--- a/fs/smb/smbdirect/rw.c
+++ b/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,