FAILED: patch "[PATCH] drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-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 <[email protected]>.

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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 1a8f537f5a1eeac941f262fe73078d6b08ba83c0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072940-harmonica-stage-8c21@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 1a8f537f5a1eeac941f262fe73078d6b08ba83c0 Mon Sep 17 00:00:00 2001
From: Ashutosh Desai <[email protected]>
Date: Sun, 10 May 2026 20:17:33 +0000
Subject: [PATCH] drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply
parsers

drm_dp_sideband_parse_remote_dpcd_read() reads num_bytes from the raw message and then unconditionally does:

  memcpy(bytes, &raw->msg[idx], num_bytes);

without checking that idx + num_bytes <= raw->curlen. raw->msg[] is 256 bytes; if a malicious or misbehaving MST hub sets num_bytes larger than the remaining payload, the memcpy reads past the received data into whatever follows in raw->msg[].

drm_dp_sideband_parse_remote_i2c_read_ack() has the same flaw (noted with a /* TODO check */ comment since the code was introduced).

Fix both functions by using a single combined check
(idx + num_bytes > curlen) before each memcpy. Since num_bytes is u8, it is always >= 0, so this strictly subsumes the simpler idx > curlen form and no separate step is needed.

index fe72245fb9ca..2ca1f2e9712c 100644
@@ -880,7 +880,7 @@ static bool drm_dp_sideband_parse_remote_dpcd_read(struct drm_dp_sideband_msg_rx
 		goto fail_len;
 	repmsg->u.remote_dpcd_read_ack.num_bytes = raw->msg[idx];
 	idx++;
-	if (idx > raw->curlen)
+	if (idx + repmsg->u.remote_dpcd_read_ack.num_bytes > raw->curlen)
 		goto fail_len;
 
 	memcpy(repmsg->u.remote_dpcd_read_ack.bytes, &raw->msg[idx], repmsg->u.remote_dpcd_read_ack.num_bytes);

@@ -916,7 +916,9 @@ static bool drm_dp_sideband_parse_remote_i2c_read_ack(struct drm_dp_sideband_msg
 		goto fail_len;
 	repmsg->u.remote_i2c_read_ack.num_bytes = raw->msg[idx];
 	idx++;
-	/* TODO check */
+	if (idx + repmsg->u.remote_i2c_read_ack.num_bytes > raw->curlen)
+		goto fail_len;
+
 	memcpy(repmsg->u.remote_i2c_read_ack.bytes, &raw->msg[idx], repmsg->u.remote_i2c_read_ack.num_bytes);
 	return true;
 fail_len:

From: Ashutosh Desai <[email protected]>

[ Upstream commit 1a8f537f5a1eeac941f262fe73078d6b08ba83c0 ]

drm_dp_sideband_parse_remote_dpcd_read() reads num_bytes from the raw message and then unconditionally does:

  memcpy(bytes, &raw->msg[idx], num_bytes);

without checking that idx + num_bytes <= raw->curlen. raw->msg[] is 256 bytes; if a malicious or misbehaving MST hub sets num_bytes larger than the remaining payload, the memcpy reads past the received data into whatever follows in raw->msg[].

drm_dp_sideband_parse_remote_i2c_read_ack() has the same flaw (noted with a /* TODO check */ comment since the code was introduced).

Fix both functions by using a single combined check
(idx + num_bytes > curlen) before each memcpy. Since num_bytes is u8, it is always >= 0, so this strictly subsumes the simpler idx > curlen form and no separate step is needed.

---
 drivers/gpu/drm/drm_dp_mst_topology.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index 70be5719e403..35604c8aeb27 100644
@@ -861,7 +861,7 @@ static bool drm_dp_sideband_parse_remote_dpcd_read(struct drm_dp_sideband_msg_rx
 		goto fail_len;
 	repmsg->u.remote_dpcd_read_ack.num_bytes = raw->msg[idx];
 	idx++;
-	if (idx > raw->curlen)
+	if (idx + repmsg->u.remote_dpcd_read_ack.num_bytes > raw->curlen)
 		goto fail_len;
 
 	memcpy(repmsg->u.remote_dpcd_read_ack.bytes, &raw->msg[idx], repmsg->u.remote_dpcd_read_ack.num_bytes);

@@ -897,7 +897,9 @@ static bool drm_dp_sideband_parse_remote_i2c_read_ack(struct drm_dp_sideband_msg
 		goto fail_len;
 	repmsg->u.remote_i2c_read_ack.num_bytes = raw->msg[idx];
 	idx++;
-	/* TODO check */
+	if (idx + repmsg->u.remote_i2c_read_ack.num_bytes > raw->curlen)
+		goto fail_len;
+
 	memcpy(repmsg->u.remote_i2c_read_ack.bytes, &raw->msg[idx], repmsg->u.remote_i2c_read_ack.num_bytes);
 	return true;
 fail_len:

2.53.0