CVE-2026-68436: drm/amd/display: use kvzalloc to allocate struct dc

[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/amd/display: use kvzalloc to allocate struct dc

struct dc has grown large over time (most of it the two inlined
dc_scratch_space copies) and now sits close to the page allocator's 4 MiB
contiguous allocation limit. Its actual size is not fixed by the source
alone, it also depends on the compiler and the .config, so it can easily
cross 4 MiB, e.g. with a newer GCC or a config change.

dc_create() allocates it with kzalloc(). Once struct dc exceeds 4 MiB the
request is rounded up to order 11 (8 MiB), which is above MAX_PAGE_ORDER,
so the page allocator warns and returns NULL. dc_create() then fails, DM
init fails and amdgpu probe aborts with -EINVAL:

  WARNING: mm/page_alloc.c:5197 at __alloc_frozen_pages_noprof+0x2f9/0x380
   dc_create+0x38/0x660 [amdgpu]
   amdgpu_dm_init+0x2d9/0x510 [amdgpu]
   dm_hw_init+0x1b/0x90 [amdgpu]
   amdgpu_device_init.cold+0x150d/0x1e13 [amdgpu]
   amdgpu_driver_load_kms+0x19/0x80 [amdgpu]
   amdgpu_pci_probe+0x1e2/0x4c0 [amdgpu]

dc_create() then returns NULL and DM init fails, which aborts the whole
GPU init and makes amdgpu probe fail with -EINVAL ("hw_init of IP block
<dm> failed -22"), leaving the display unusable. The subsequent
amdgpu_irq_put() warnings during teardown are just fallout of unwinding
a half-initialized device.

struct dc is a software-only bookkeeping structure that is never handed
to hardware DMA and is only ever kept as an opaque pointer, so it does
not require physically contiguous memory. Allocate it with kvzalloc()
(and free it with kvfree()) so that the allocator can fall back to
vmalloc() when a contiguous allocation of that size is not available,
which also avoids the MAX_PAGE_ORDER warning entirely.

v2:
 - Rebase to amd-staging-drm-next.

(cherry picked from commit 991e0516a8072f2292681c6ae98a924ab0e32575)

The Linux kernel CVE team has assigned CVE-2026-68436 to this issue.


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

	Fixed in 7.1.6 with commit dbad70d40cad9c5e7586953275287fe7531fb811
	Fixed in 7.2-rc2 with commit 75050390151a14802be433c3856ddcb483cecd24

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-2026-68436
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/amd/display/dc/core/dc.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/dbad70d40cad9c5e7586953275287fe7531fb811
	https://git.kernel.org/stable/c/75050390151a14802be433c3856ddcb483cecd24