CVE-2025-71305: drm/display/dp_mst: Add protection against 0 vcpi

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

drm/display/dp_mst: Add protection against 0 vcpi

When releasing a timeslot there is a slight chance we may end up
with the wrong payload mask due to overflow if the delayed_destroy_work
ends up coming into play after a DP 2.1 monitor gets disconnected
which causes vcpi to become 0 then we try to make the payload =
~BIT(vcpi - 1) which is a negative shift. VCPI id should never
really be 0 hence skip changing the payload mask if VCPI is 0.

Otherwise it leads to
<7> [515.287237] xe 0000:03:00.0: [drm:drm_dp_mst_get_port_malloc
[drm_display_helper]] port ffff888126ce9000 (3)
<4> [515.287267] -----------[ cut here ]-----------
<3> [515.287268] UBSAN: shift-out-of-bounds in
../drivers/gpu/drm/display/drm_dp_mst_topology.c:4575:36
<3> [515.287271] shift exponent -1 is negative
<4> [515.287275] CPU: 7 UID: 0 PID: 3108 Comm: kworker/u64:33 Tainted: G
S U 6.17.0-rc6-lgci-xe-xe-3795-3e79699fa1b216e92+ #1 PREEMPT(voluntary)
<4> [515.287279] Tainted: [S]=CPU_OUT_OF_SPEC, [U]=USER
<4> [515.287279] Hardware name: ASUS System Product Name/PRIME Z790-P
WIFI, BIOS 1645 03/15/2024
<4> [515.287281] Workqueue: drm_dp_mst_wq drm_dp_delayed_destroy_work
[drm_display_helper]
<4> [515.287303] Call Trace:
<4> [515.287304] <TASK>
<4> [515.287306] dump_stack_lvl+0xc1/0xf0
<4> [515.287313] dump_stack+0x10/0x20
<4> [515.287316] __ubsan_handle_shift_out_of_bounds+0x133/0x2e0
<4> [515.287324] ? drm_atomic_get_private_obj_state+0x186/0x1d0
<4> [515.287333] drm_dp_atomic_release_time_slots.cold+0x17/0x3d
[drm_display_helper]
<4> [515.287355] mst_connector_atomic_check+0x159/0x180 [xe]
<4> [515.287546] drm_atomic_helper_check_modeset+0x4d9/0xfa0
<4> [515.287550] ? __ww_mutex_lock.constprop.0+0x6f/0x1a60
<4> [515.287562] intel_atomic_check+0x119/0x2b80 [xe]
<4> [515.287740] ? find_held_lock+0x31/0x90
<4> [515.287747] ? lock_release+0xce/0x2a0
<4> [515.287754] drm_atomic_check_only+0x6a2/0xb40
<4> [515.287758] ? drm_atomic_add_affected_connectors+0x12b/0x140
<4> [515.287765] drm_atomic_commit+0x6e/0xf0
<4> [515.287766] ? _pfx__drm_printfn_info+0x10/0x10
<4> [515.287774] drm_client_modeset_commit_atomic+0x25c/0x2b0
<4> [515.287794] drm_client_modeset_commit_locked+0x60/0x1b0
<4> [515.287795] ? mutex_lock_nested+0x1b/0x30
<4> [515.287801] drm_client_modeset_commit+0x26/0x50
<4> [515.287804] __drm_fb_helper_restore_fbdev_mode_unlocked+0xdc/0x110
<4> [515.287810] drm_fb_helper_hotplug_event+0x120/0x140
<4> [515.287814] drm_fbdev_client_hotplug+0x28/0xd0
<4> [515.287819] drm_client_hotplug+0x6c/0xf0
<4> [515.287824] drm_client_dev_hotplug+0x9e/0xd0
<4> [515.287829] drm_kms_helper_hotplug_event+0x1a/0x30
<4> [515.287834] drm_dp_delayed_destroy_work+0x3df/0x410
[drm_display_helper]
<4> [515.287861] process_one_work+0x22b/0x6f0
<4> [515.287874] worker_thread+0x1e8/0x3d0
<4> [515.287879] ? __pfx_worker_thread+0x10/0x10
<4> [515.287882] kthread+0x11c/0x250
<4> [515.287886] ? __pfx_kthread+0x10/0x10
<4> [515.287890] ret_from_fork+0x2d7/0x310
<4> [515.287894] ? __pfx_kthread+0x10/0x10
<4> [515.287897] ret_from_fork_asm+0x1a/0x30

The Linux kernel CVE team has assigned CVE-2025-71305 to this issue.


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

	Fixed in 6.1.165 with commit 95dbd525efce2a9e9e1c50ad15213de644c85ad0
	Fixed in 6.6.128 with commit ac9a7c329a5610051fc476644c9b9145a5965ecb
	Fixed in 6.12.75 with commit 3f44cdb5371faf225af37d5caba8f21ec0572469
	Fixed in 6.18.16 with commit 4d2ccdea18b564e3f73e3e543854acea64e6277d
	Fixed in 6.19.4 with commit d6afc7539ce06dadfa5b4787b3cfe79b95d8f67a
	Fixed in 7.0 with commit 342ccffd9f77fc29fe1c05fd145e4d842bd2feaa

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-2025-71305
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/95dbd525efce2a9e9e1c50ad15213de644c85ad0
	https://git.kernel.org/stable/c/ac9a7c329a5610051fc476644c9b9145a5965ecb
	https://git.kernel.org/stable/c/3f44cdb5371faf225af37d5caba8f21ec0572469
	https://git.kernel.org/stable/c/4d2ccdea18b564e3f73e3e543854acea64e6277d
	https://git.kernel.org/stable/c/d6afc7539ce06dadfa5b4787b3cfe79b95d8f67a
	https://git.kernel.org/stable/c/342ccffd9f77fc29fe1c05fd145e4d842bd2feaa