[CVE-2026-63884][IMPORTANT] drm/i915: Fix potential UAF in TTM object purge

[CVE-2026-63884][IMPORTANT] drm/i915: Fix potential UAF in TTM object purge

[AL-KERNEL]

CVE: CVE-2026-63884
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/i915: Fix potential UAF in TTM object purge
Commit: df73f3bc731af1c39ac5405bc59c4e7c6f8e9117
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df73f3bc731af1c39ac5405bc59c4e7c6f8e9117
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63884
Analysis date: Sun, 19 Jul 2026 12:05:53 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale pointer use-after-free in the i915 TTM purge path can dereference and operate on an outdated `i915_ttm_tt` object after `ttm_bo_validate()` replaces `bo->ttm`, causing a local kernel crash and requiring manual review for possible privilege-escalation impact.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-63884 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63884

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-63884	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stale pointer use after free can occur in the i915 TTM purge path because i915_ttm_purge() captured the i915 TTM page vector container before a validation step that may replace the underlying TTM object. Later code may dereference the old container and operate on its filp field, which can lead to a general protection fault and kernel crash under heavy GPU memory pressure or eviction activity. For the CVSS the PR:L is selected because reliable triggering requires local access to DRM/i915 GPU interfaces, but this access is commonly available to an unprivileged desktop user or a user allowed to use the render node. The issue is not network reachable and requires local code execution on a system using the affected i915 TTM path. Impact is at least local denial of service via kernel crash, and in the paranoid interpretation it may allow confidentiality or integrity impact because the bug is a UAF involving a stale kernel object pointer rather than a simple NULL dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-63884 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB UAF KERNEL_PANIC_PLUS_UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The path is reachable through DRM/i915 GPU userspace activity, including GEM execbuffer and memory pressure paths. On many desktop systems, DRM render node access is available to non-root users or users in the render/video group.

* Generic memory corruption, strong primitive: +2
  The bug is a stale pointer / UAF class issue. `i915_ttm_purge()` captured an `i915_ttm_tt` pointer before `ttm_bo_validate()`, but `ttm_bo_validate()` may replace `bo->ttm`, making the earlier container pointer invalid.

* Real lifetime corruption: +1
  This is a real object lifetime bug: a pointer to a TTM-backed container may survive replacement of the underlying object and later be dereferenced.

* Weak LPE concern: +1
  The stale object is not merely read. The code may dereference `i915_tt->filp`, call `file_inode()`, `shmem_truncate_range()`, and perform `fput(fetch_and_zero(&i915_tt->filp))`. There is no demonstrated controlled reclaim or arbitrary write, so this is not strong LPE plausibility, but it is enough for manual escalation concern.

* Reliable kernel crash / strong DoS: +1
  The commit includes a real Oops / general protection fault in `i915_ttm_purge()` under heavy GPU usage.

* Broad/default/common subsystem exposure: +1
  i915 is a common desktop/workstation graphics driver on affected Intel GPU systems. The affected path is not an obscure debug-only interface.

* Local high-control device object API refinement: +1, paranoid only
  DRM/GEM userspace can shape buffer-object lifetimes, memory pressure, eviction, and validation paths. This is not counted in the conservative score as a full high-control data-plane bonus, but it supports the paranoid score.

Negative signals:

* Special load / timing / hardware condition: -1
  The observed crash happened under heavy use on DG2 and depends on TTM eviction/purge behavior. This reduces confidence in trivial reliable exploitation.

Conservative total: 6
Paranoid total: 7

## 3. Reachability analysis

A local user or process with access to DRM/i915 interfaces can plausibly trigger the affected path by creating GPU buffer activity that causes TTM validation, eviction, and purge. The stack trace shows the path through `i915_gem_execbuffer2_ioctl()`, `i915_vma_pin_ww()`, `i915_ttm_get_pages()`, `ttm_bo_validate()`, eviction, and `i915_ttm_purge()`.

This is not network reachable. Containers only matter if the DRM device is exposed into the container. In that case, a containerized user with access to the render node may have practical reachability even without host root.

The path is enabled on affected i915 TTM configurations and is realistic on Intel GPU systems using this driver path, but the trigger appears workload-dependent and tied to eviction or memory pressure.

Call-site confidence: high. The patch, stack trace, and surrounding call path are provided.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a real stale pointer / UAF class bug in a common local GPU driver path, with a demonstrated kernel crash and a plausible but not proven privilege-escalation concern.

The realistic demonstrated impact is local DoS through kernel crash. The theoretical higher impact comes from stale kernel object use involving a file pointer field and `fetch_and_zero()`, but the patch does not show attacker-controlled reclaim, controlled replacement, callback control, or an arbitrary write primitive. Therefore, the conservative score is Actionable Moderate, while the paranoid score reaches Strong Important candidate for manual review.

## 5. One-sentence report phrase

A stale pointer use-after-free in the i915 TTM purge path can dereference and operate on an outdated `i915_ttm_tt` object after `ttm_bo_validate()` replaces `bo->ttm`, causing a local kernel crash and requiring manual review for possible privilege-escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a UAF/stale pointer lifetime bug in a common DRM/i915 path with a real crash trace and plausible kernel object corruption concerns, even though no controlled reclaim or reliable LPE primitive is demonstrated.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: drm/i915: Fix potential UAF in TTM object purge
Commit: df73f3bc731af1c39ac5405bc59c4e7c6f8e9117
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df73f3bc731af1c39ac5405bc59c4e7c6f8e9117

Commit description:

TLDR: The bo->ttm object might be changed by calling ttm_bo_validate(),
      move casting it to an i915_tt object later to actually get the right
      pointer.

A user reported hitting the following bug under heavy use on DG2:

[26620.095550] Oops: general protection fault, probably for non-canonical address 0xa56b6b6b6b6b6b8b: 0000 1 SMP NOPTI
[26620.095556] CPU: 2 UID: 0 PID: 631 Comm: Xorg Not tainted 6.18.8 #1 PREEMPT(lazy)
[26620.095558] Hardware name: ASRock B850M Steel Legend WiFi/B850M Steel Legend WiFi, BIOS 3.50 09/18/2025
[26620.095559] RIP: 0010:i915_ttm_purge+0x84/0x100 [i915]
[26620.095604] Code: 00 00 00 48 8d 54 24 10 48 89 e6 48 89 fb e8 83 aa ae ff 85 c0 75 6f 48 83 bb a8 01 00 00 00 74 2c 48 8b 45 78 48 85 c0 74 23 <48> 8b 78 20 48 c7 c2 ff ff ff ff 31 f6 e8 7a 73 e3 e0 48 8b 7d 78
[26620.095605] RSP: 0018:ffffc90005fd7430 EFLAGS: 00010282
[26620.095607] RAX: a56b6b6b6b6b6b6b RBX: ffff8881f46c3dc0 RCX: 0000000000000000
[26620.095608] RDX: 0000000000000000 RSI: 0000000000000246 RDI: 00000000ffffffff
[26620.095609] RBP: ffff888289610f00 R08: 0000000000000001 R09: ffff88823b022000
[26620.095609] R10: ffff888103029b28 R11: ffff8881fc7f3800 R12: ffff88810b6150d0
[26620.095609] R13: ffff888289610f00 R14: 0000000000000000 R15: ffff8881f46c3dc0
[26620.095610] FS: 00007f1004d86900(0000) GS:ffff88901c858000(0000) knlGS:0000000000000000
[26620.095611] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[26620.095611] CR2: 00007f0fdf489000 CR3: 000000035b0c1000 CR4: 0000000000750ef0
[26620.095612] PKRU: 55555554
[26620.095612] Call Trace:
[26620.095615] <TASK>
[26620.095615] i915_ttm_move+0x2b9/0x420 [i915]
[26620.095642] ? ttm_tt_init+0x65/0x80 [ttm]
[26620.095644] ? i915_ttm_tt_create+0xc6/0x150 [i915]
[26620.095667] ttm_bo_handle_move_mem+0xb6/0x160 [ttm]
[26620.095669] ttm_bo_evict+0x100/0x150 [ttm]
[26620.095671] ? preempt_count_add+0x64/0xa0
[26620.095673] ? _raw_spin_lock+0xe/0x30
[26620.095675] ? _raw_spin_unlock+0xd/0x30
[26620.095675] ? i915_gem_object_evictable+0xb7/0xd0 [i915]
[26620.095704] ttm_bo_evict_cb+0x6e/0xd0 [ttm]
[26620.095705] ttm_lru_walk_for_evict+0xa6/0x200 [ttm]
[26620.095708] ttm_bo_alloc_resource+0x185/0x4f0 [ttm]
[26620.095709] ? init_object+0x62/0xd0
[26620.095712] ttm_bo_validate+0x7a/0x180 [ttm]
[26620.095713] ? _raw_spin_unlock_irqrestore+0x16/0x30
[26620.095714] __i915_ttm_get_pages+0xb0/0x170 [i915]
[26620.095737] i915_ttm_get_pages+0x9f/0x150 [i915]
[26620.095759] ? i915_gem_do_execbuffer+0xedc/0x2b40 [i915]
[26620.095786] ? alloc_debug_processing+0xd0/0x100
[26620.095787] ? _raw_spin_unlock_irqrestore+0x16/0x30
[26620.095788] ? i915_vma_instance+0xa0/0x4e0 [i915]
[26620.095822] __i915_gem_object_get_pages+0x2f/0x40 [i915]
[26620.095848] i915_vma_pin_ww+0x706/0x980 [i915]
[26620.095875] ? i915_gem_do_execbuffer+0xedc/0x2b40 [i915]
[26620.095904] eb_validate_vmas+0x170/0xa00 [i915]
[26620.095930] i915_gem_do_execbuffer+0x1201/0x2b40 [i915]
[26620.095953] ? alloc_debug_processing+0xd0/0x100
[26620.095954] ? _raw_spin_unlock_irqrestore+0x16/0x30
[26620.095955] ? i915_gem_execbuffer2_ioctl+0xc9/0x240 [i915]
[26620.095977] ? __wake_up_sync_key+0x32/0x50
[26620.095979] ? i915_gem_execbuffer2_ioctl+0xc9/0x240 [i915]
[26620.096001] ? __slab_alloc.isra.0+0x67/0xc0
[26620.096003] i915_gem_execbuffer2_ioctl+0x11a/0x240 [i915]

Results from decode_stacktrace.sh pointed to dereference of a file pointer
field of a i915 TTM page vector container associated with an object being
purged on eviction.  That path is taken when the object is marked as no
longer needed.

Code analysis revealed a possibility of the i915 TTM page vector container
being replaced with a new instance inside a function that purges content
of the object, should it be still busy.  That function is called,
indirectly via a more general function that changes the object's placement
and caching policy, before the problematic dereference, but still after
a pointer to the container is captured, rendering the pointer no longer
valid.

Fix the issue by capturing the pointer to the container only after its
potential replacement.

v2: Move the container_of() inside the if block (Sebastian),
  - a simplified version of the commit description that explains briefly
    why the change is necessary (Christian).

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/14882
Fixes: 7ae0345 ("drm/i915/ttm: add tt shmem backend")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>

[Commit description truncated; see the upstream URL below]

Changed files:
  drivers/gpu/drm/i915/gem/i915_gem_ttm.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df73f3bc731af1c39ac5405bc59c4e7c6f8e9117

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-63884 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63884

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 6
  Paranoid ActionableScore: 7

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).