CVE-2026-46209: drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

CVE-2026-46209: drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

[Greg Kroah-Hartman]

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

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

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

drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

drm_gem_fb_init_with_funcs() computes sub-sampled plane dimensions
using plain integer division:

  unsigned int width  = mode_cmd->width  / (i ? info->hsub : 1);
  unsigned int height = mode_cmd->height / (i ? info->vsub : 1);

However, the ioctl-level framebuffer_check() in drm_framebuffer.c uses
drm_format_info_plane_width/height() which round up dimensions via
DIV_ROUND_UP(). This inconsistency corrupts the subsequent GEM object
size check for certain pixel format and dimension combinations.

For example, with NV12 (vsub=2) and a 1-pixel-tall framebuffer the
GEM size validation path sees height=0 instead of height=1. The
expression (height - 1) then wraps to UINT_MAX as an unsigned int,
causing min_size to overflow and wrap back to a small value. A tiny
GEM object therefore passes the size guard, yet when the GPU accesses
the chroma plane it will read or write memory beyond the object's
bounds.

Fix by replacing the open-coded divisions with drm_format_info_plane_width()
and drm_format_info_plane_height(), which use DIV_ROUND_UP() and match
the calculation already used in framebuffer_check().

The Linux kernel CVE team has assigned CVE-2026-46209 to this issue.


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

	Issue introduced in 4.14 with commit 4c3dbb2c312c9fafbac30d98c523b8b1f3455d78 and fixed in 6.6.140 with commit 6b992591e04f2cce813bcf239b354f375bbf84d3
	Issue introduced in 4.14 with commit 4c3dbb2c312c9fafbac30d98c523b8b1f3455d78 and fixed in 6.12.90 with commit 1da4ab7189f1064b3b712b388772c008b4d82580
	Issue introduced in 4.14 with commit 4c3dbb2c312c9fafbac30d98c523b8b1f3455d78 and fixed in 6.18.32 with commit 1a17ea9861e89585361caa8bc231bd22dc6dbe7d
	Issue introduced in 4.14 with commit 4c3dbb2c312c9fafbac30d98c523b8b1f3455d78 and fixed in 7.0.9 with commit c5fc49d8470c5ebf3b41607600f277158f159950
	Issue introduced in 4.14 with commit 4c3dbb2c312c9fafbac30d98c523b8b1f3455d78 and fixed in 7.1-rc2 with commit 3d4c2268bd7243c3780fe32bf24ff876da272acf

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-2026-46209
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/drm_gem_framebuffer_helper.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/6b992591e04f2cce813bcf239b354f375bbf84d3
	https://git.kernel.org/stable/c/1da4ab7189f1064b3b712b388772c008b4d82580
	https://git.kernel.org/stable/c/1a17ea9861e89585361caa8bc231bd22dc6dbe7d
	https://git.kernel.org/stable/c/c5fc49d8470c5ebf3b41607600f277158f159950
	https://git.kernel.org/stable/c/3d4c2268bd7243c3780fe32bf24ff876da272acf