[PATCH v2] drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply parsers

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

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.

Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
Changes in v2:
- Drop separate idx > curlen check; idx + num_bytes > curlen with u8
  num_bytes (always >= 0) strictly subsumes it (Lyude Paul)

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

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index 170113520a43..9416a48804c8 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -871,7 +871,7 @@ 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);
@@ -907,7 +907,9 @@ 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:
-- 
2.34.1