[PATCH] drm/dp/mst: fix buffer overflows in sideband chunk accumulation

Ashutosh Desai
2026-04-10T07:07:00Z

drm_dp_sideband_append_payload() has three related bugs when processing
device-provided sideband reply data:

1. Zero-length curchunk_len underflow: msg_len is a 6-bit field taken
directly from the DP sideband header. If a device sends msg_len=0,
curchunk_len is set to zero. The condition (curchunk_idx >= curchunk_len)
is immediately true, and curchunk_len-1 wraps to 255 (u8 underflow).
drm_dp_msg_data_crc4() reads 255 bytes from chunk[48], then memcpy()
writes 255 bytes into msg[], both far out of bounds.

2. chunk[48] overflow: curchunk_len can reach 63 (6-bit field). chunk[] is
only 48 bytes. Multi-iteration payload assembly appends 16-byte blocks
until curchunk_idx reaches curchunk_len, writing up to 15 bytes past
the end of chunk[] into msg[].

3. msg[256] overflow: each chunk contributes (curchunk_len-1) bytes to
msg[]. No check ensures curlen + (curchunk_len-1) stays within msg[256],
so the memcpy can spill into adjacent struct fields.

All three are reachable from any DP MST device that can forge sideband
reply messages on a physical connection.

Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
drivers/gpu/drm/display/drm_dp_mst_topology.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index f2a7dbc5e..5261a4a54 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -789,6 +789,12 @@ static bool drm_dp_sideband_append_payload(struct drm_dp_sideband_msg_rx *msg,
{
u8 crc4;

+ /* curchunk_len must be >= 1 (min 1 CRC byte) and fit in chunk[] */
+ if (!msg->curchunk_len ||
+ msg->curchunk_len > ARRAY_SIZE(msg->chunk) ||
+ msg->curchunk_idx + replybuflen > ARRAY_SIZE(msg->chunk))
+ return false;
+
memcpy(&msg->chunk[msg->curchunk_idx], replybuf, replybuflen);
msg->curchunk_idx += replybuflen;

@@ -799,6 +805,9 @@ static bool drm_dp_sideband_append_payload(struct drm_dp_sideband_msg_rx *msg,
print_hex_dump(KERN_DEBUG, "wrong crc",
DUMP_PREFIX_NONE, 16, 1,
msg->chunk, msg->curchunk_len, false);
+ /* Guard against accumulated msg[] overflow */
+ if (msg->curlen + msg->curchunk_len - 1 > ARRAY_SIZE(msg->msg))
+ return false;
/* copy chunk into bigger msg */
memcpy(&msg->msg[msg->curlen], msg->chunk, msg->curchunk_len - 1);
msg->curlen += msg->curchunk_len - 1;
--
2.34.1