CVE-2026-98521: drm/i915: fix incorrect RCU teardown order

[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/i915: fix incorrect RCU teardown order

i915_gem_busy_ioctl uses dma_resv_for_each_fence_unlocked() to iterate
over the fences in an GEM object without holding a reference but only
the RCU read side lock.

What can happen here is that the GEM object is destroyed concurrently
while i915_gem_busy_ioctl is still running. This won't free the GEM
objects memory, but still drops all the dma_fence references.

Now when dma_resv_for_each_fence_unlocked() sees a destroyed dma_fence it
assumes that a new fence list was installed and re-starts the loop.

But in the case of a destroyed GEM object a new fence list is never
installed, only the old one freed and therefore the iteration never
finishes resulting in an endless loop.

The solution is to drop the fence references only after the RCU grace
period.

The fixes tag is not necessary the patch introducing the problem, but the
one making it so worse that we need to address it.

This problem was pointed out by Sashiko-bot.

(cherry picked from commit 5113479556025093bf8133bb2dcaa33be2d50921)

The Linux kernel CVE team has assigned CVE-2026-98521 to this issue.


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

	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 6.1.189 with commit 4d4ca84213327da1f84ef7af3946dea6b14c5d88
	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 6.6.158 with commit c53db9cc245219af6b528ca022a17ebfa91d80ba
	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 6.12.112 with commit b12e041aedcf9fa77c6002a49f07eb20d0521b86
	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 6.18.55 with commit cc890d3f1b15bf6481dc7270fba0b4fbba572813
	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 7.2.9 with commit 2dd0190e397ec6468095fd3e80e79276a7361449
	Issue introduced in 5.17 with commit 912ff2ebd695c582ca4cebf833ad73a378a99f23 and fixed in 7.3-rc5 with commit d2da6696e0c4e60414706e607029d0bb0330c67e

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-2026-98521
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/i915/gem/i915_gem_object.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/4d4ca84213327da1f84ef7af3946dea6b14c5d88
	https://git.kernel.org/stable/c/c53db9cc245219af6b528ca022a17ebfa91d80ba
	https://git.kernel.org/stable/c/b12e041aedcf9fa77c6002a49f07eb20d0521b86
	https://git.kernel.org/stable/c/cc890d3f1b15bf6481dc7270fba0b4fbba572813
	https://git.kernel.org/stable/c/2dd0190e397ec6468095fd3e80e79276a7361449
	https://git.kernel.org/stable/c/d2da6696e0c4e60414706e607029d0bb0330c67e