CVE-2026-99000 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/xe: Fix shrinker accounting double-subtraction on nested external pins
xe_bo_pin_external() and xe_bo_unpin_external() support nested pins, since the underlying ttm_bo_pin()/ttm_bo_unpin() maintain a pin_count refcount rather than a boolean. However, the shrinker accounting calls xe_ttm_tt_account_subtract() and xe_ttm_tt_account_add() are invoked unconditionally on every pin_external()/unpin_external() call, instead of only on the transition into and out of the pinned state.
An external BO (dma-buf) can be pinned more than once while already pinned, for example when it has multiple attachments/importers, each independently calling xe_bo_pin_external(). Each such nested pin subtracts the BO's pages from the shrinker's accounting again, even though the BO's pages were already removed from consideration by the first pin. Symmetrically, each nested unpin adds them back again before the BO is actually unpinned. This desynchronizes the shrinker's page and object counts from reality, and since they are declared as signed long but interpreted as unsigned long in xe_shrinker_count(), the underflow can turn into an enormous shrinkable/purgeable page count, causing the shrinker to be invoked excessively under memory pressure.
Guard the accounting calls with the same pin-count transition checks already used to guard the pinned_link list maintenance, so accounting is only updated on the outermost pin and the final unpin.
While centralizing this logic into xe_bo_account_pin()/ xe_bo_account_unpin(), also close a second, related accounting gap affecting imported dma-bufs. For an imported bo's ttm_tt, xe's ttm_tt_populate hook (xe_ttm_tt_populate()) early-returns without calling xe_ttm_tt_account_add(), since TTM_TT_FLAG_EXTERNAL is set without TTM_TT_FLAG_EXTERNAL_MAPPABLE. However, TTM core's ttm_tt_populate() wrapper unconditionally marks the tt as populated on driver-hook success, so ttm_tt_is_populated() still reports true for it. xe_bo_account_pin()/xe_bo_account_unpin() trusted ttm_tt_is_populated() as a proxy for "these pages are accounted for by the shrinker", and would subtract/add pages for such a tt that were never added in the first place, again underflowing the shrinker's counts. This is reachable today via xe_bo_pin_external(), and would have become newly reachable via the fb-pin path once this patch routes __xe_pin_fb_vma() through xe_bo_pin_account() below, so fix it here rather than in a later patch, to avoid a commit in this series that introduces the regression before fixing it. Skip the accounting for imported bos, using the same EXTERNAL && !EXTERNAL_MAPPABLE test xe_ttm_tt_populate()/xe_ttm_tt_unpopulate() use to decide whether to act, via a new xe_ttm_bo_is_imported() helper (hoisted up from its only prior use site).
v2: - Fix the critical issue caught by Sashiko AI review: the pin-count transition check introduced here was still bypassed by the fb-pin path (__xe_pin_fb_vma()/__xe_unpin_fb_vma()), which could leave a stale pinned_link entry and a permanent shrinker accounting leak when the fb-unpin performed the true last unpin. Addressed in a separate follow-up patch in this series rather than folded in here, since it is a distinct bug with its own Fixes: tag. - Drop the unneeded local "last_unpin" variable in xe_bo_unpin_external(), restoring the original inline bo->ttm.pin_count == 1 check; it was a no-op rename not used by any of the new accounting helpers, and was going to be deleted again by the follow-up patch anyway.
v3: - Exclude imported dma-bufs from the pin/unpin shrinker accounting, fixing another underflow caught by Sashiko AI review. - Add a short comment on the backup-object pin site noting it's already populated/accounted at pin time, addressing (as a false-positive clarification, no code fix needed) a separate Medium-severity accounting-leak concern also caught by Sashiko AI ---truncated---
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Analysis
by VulDB Data Team • 10/24/2026
The Linux kernel driver for Intel Xe graphics contains a critical logic error in the buffer object pinning and unpinning subsystem that leads to severe memory management desynchronization. The vulnerability stems from an incorrect implementation of shrinker accounting within xe_bo_pin_external() and xe_bo_unpin_external(). These functions are designed to support nested pins, leveraging the underlying ttm_bo_pin() and ttm_bo_unpin() mechanisms which maintain a pin_count reference count rather than a simple boolean state. This architecture allows external buffer objects, such as dma-bufs, to be pinned multiple times by different attachments or importers while already in a pinned state. However, the shrinker accounting functions xe_ttm_tt_account_subtract() and xe_ttm_tt_account_add() were invoked unconditionally on every call to pin_external or unpin_external, regardless of whether this represented a transition into or out of the actual pinned state for that specific instance.
This unconditional invocation causes significant data corruption within the kernel's memory shrinker subsystem. When an external buffer object is nested-pinned multiple times, each subsequent pin incorrectly subtracts the associated pages from the shrinker accounting again, even though those pages were already removed from consideration by the initial pin operation. Conversely, during unpinning, each nested unpin erroneously adds the pages back into the account before the final unpin occurs. This results in a desynchronization between the shrinker's internal page and object counts and the actual state of system memory. Because these counters are declared as signed long integers but interpreted as unsigned long values within xe_shrinker_count(), an underflow condition can occur where negative values wrap around to extremely large positive numbers.
The operational impact of this vulnerability is severe, particularly under conditions of high memory pressure. The inflated shrinkable and purgeable page counts trick the kernel into believing there is significantly more reclaimable memory than actually exists. Consequently, the memory shrinker may be invoked excessively and aggressively, attempting to free pages that are still actively pinned by hardware or other drivers. This can lead to performance degradation, system instability, or potential denial of service as the kernel wastes resources on futile reclamation attempts while potentially disrupting legitimate memory operations for active graphics contexts.
A secondary aspect of this vulnerability involves imported dma-bufs where TTM_TT_FLAG_EXTERNAL is set without TTM_TT_FLAG_EXTERNAL_MAPPABLE. In these cases, xe_ttm_tt_populate() returns early without calling the accounting addition function, yet ttm_tt_is_populated() still reports true because it relies on driver hook success rather than actual accounting status. The previous logic trusted this populated flag as a proxy for accounted pages, leading to further underflows when pinning such buffers. This issue was exacerbated by changes in framebuffer pin paths that routed through the affected functions without proper guards.
The resolution involves restructuring the accounting logic to align strictly with state transitions rather than function calls. By introducing checks similar to those used for pinned_link list maintenance, the patch ensures that accounting adjustments only occur on the outermost pin and the final unpin operations. Additionally, a new helper xe_ttm_bo_is_imported() is utilized to exclude imported dma-bufs from this specific shrinker accounting path, preventing the secondary underflow scenario. These changes restore consistency between the kernel's memory management views and the actual hardware state, ensuring stable operation during graphics-intensive workloads that rely on buffer sharing across different drivers or subsystems.