CVE-2026-68278: drm/dp/mst: fix buffer overflows in sideband chunk accumulation

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

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

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.

The Linux kernel CVE team has assigned CVE-2026-68278 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.17 with commit ad7f8a1f9ced7f049f9b66d588723f243a7034cd and fixed in 6.6.148 with commit 53937a2787d29c7a460e984dc4f20ff6ac91dc65
	Issue introduced in 3.17 with commit ad7f8a1f9ced7f049f9b66d588723f243a7034cd and fixed in 6.12.101 with commit ef0dbcc200c3389f1f781ab181932a97e54b51af
	Issue introduced in 3.17 with commit ad7f8a1f9ced7f049f9b66d588723f243a7034cd and fixed in 6.18.42 with commit 1e5827839ad0ceb0079d1560c321fa3656b54f21
	Issue introduced in 3.17 with commit ad7f8a1f9ced7f049f9b66d588723f243a7034cd and fixed in 7.1.6 with commit a6366b551079c79bf7bdbadd74c97358bcfe2d58
	Issue introduced in 3.17 with commit ad7f8a1f9ced7f049f9b66d588723f243a7034cd and fixed in 7.2-rc1 with commit 55bd5e685bda455b9b50c835f8c8442d52a344a3

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-68278
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/gpu/drm/display/drm_dp_mst_topology.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/53937a2787d29c7a460e984dc4f20ff6ac91dc65
	https://git.kernel.org/stable/c/ef0dbcc200c3389f1f781ab181932a97e54b51af
	https://git.kernel.org/stable/c/1e5827839ad0ceb0079d1560c321fa3656b54f21
	https://git.kernel.org/stable/c/a6366b551079c79bf7bdbadd74c97358bcfe2d58
	https://git.kernel.org/stable/c/55bd5e685bda455b9b50c835f8c8442d52a344a3