CVE-2026-68266: drm/xe: Hold a dma-buf reference for imported BOs

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

drm/xe: Hold a dma-buf reference for imported BOs

An imported dma-buf BO is created as a ttmbotypesg BO whose reservation object is the exporter's dmabuf->resv. The importer, however, only takes a dma-buf reference after a successful dmabufdynamicattach(). Until then nothing keeps the exporter alive, so if the exporter is freed while the BO still references its resv, a later access to that resv is a use-after-free:

Oops: general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b9c Workqueue: ttm ttmbodelayeddelete [ttm] RIP: 0010:mutexcanspinonowner+0x3f/0xc0

This can be reached on two paths:

- dmabufdynamicattach() fails, or - ttmboinitreserved() fails during BO creation.

In both cases the BO already has bo->base.resv pointing at the exporter resv, and sg BOs are always torn down via ttmbodelayeddelete(), which locks bo->base.resv asynchronously - potentially after the exporter has been freed.

Take the dma-buf reference in xeboinitlocked(), before ttmboinitreserved(), so it also covers a creation failure there, and release it in xettmbodestroy(). The reference is held for the whole BO lifetime, keeping the shared resv alive on every path.

v2: - Reworked the fix to avoid creating the imported sg BO before dmabufdynamicattach() succeeds. - Attach with importerpriv == NULL and make invalidatemappings ignore incomplete imports.

v3: - Dropped the xe-side reordering approach since importerpriv must be valid when dmabufdynamicattach() publishes the attachment. - Per Christian's suggestion on the v1 thread, keyed the check on importattach rather than removing the sg guard entirely. - Fixes both xe and amdgpu in a single TTM patch.

v4: - Moved importattach check to after dmaresvcopyfences() so fences are copied before returning for successful imports (Thomas). - Removed exporter-alive claim from commit message (Thomas).

v5: - Add drm/xe patch to keep imported sg BOs off the LRU before attach succeeds; the TTM fix alone is not sufficient for xe if the BO is already LRU-visible. (Thomas) v4 patch: https://patchwork.freedesktop.org/patch/736663/?series=169129&rev=2 - Patch 1 (drm/ttm) carries Christian's Reviewed-by from v4.

v6: - Reworked the fix based on Thomas' suggestion. Instead of the TTM resv individualization (v1-v5) plus the xe off-LRU/placement handling (v5), just hold a dma-buf reference for the imported BO lifetime so the shared resv can never be freed while the BO still references it. Single xe patch, no TTM change. (Thomas) - Take the reference in xeboinitlocked() before ttmboinitreserved() so a TTM creation failure is covered too (Thomas). - Dropped the v5 series (drm/ttm + drm/xe off-LRU); the off-LRU approach also regressed in CI BAT via ttmbopipelinegutting() creating a ghost BO that outlived the exporter. Link to v5: https://patchwork.freedesktop.org/series/169984/

v7: - Move changelog above --- so it stays in the commit message. - Reorder changelog entries oldest-to-newest. (Thomas)

(cherry picked from commit 3516f3fae6be35642f8f06f8a218da6425c0306a)