CVE-2026-89815 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/ttm: Drop tt->restore after successful restore
ttm_pool_restore_and_alloc() can successfully complete the restore process via ttm_pool_restore_commit(), but tt->restore is not dropped afterward. As a result, subsequent backup/restore flows observe what appears to be a completed restore, while in reality shmem handles are still installed in tt->pages, leading to the stack trace below.
Fix this by freeing and dropping tt->restore in ttm_pool_restore_and_alloc() upon successful completion of the restore.
20545 [ 309.784531] RIP: 0010:sg_alloc_append_table_from_pages+0x38c/0x490
20547 [ 309.809570] RSP: 0018:ffffc9000623b838 EFLAGS: 00010206
20548 [ 309.814827] RAX: 0000000000001000 RBX: ffff88816e42a160 RCX: 0000000000000000
20549 [ 309.821986] RDX: 0000000000002000 RSI: 0000000000000003 RDI: 0000000000001000
20550 [ 309.829147] RBP: ffff88816e42a168 R08: 0000000000000002 R09: 000000007ffff000
20551 [ 309.836310] R10: ffffc9000623b928 R11: 0000000000000000 R12: 000000007ffff000
20552 [ 309.843471] R13: ffff88815ba5a100 R14: 0000000000000000 R15: 0000000000000001
20553 [ 309.850634] FS: 00007f9ff305e700(0000) GS:ffff888276c94000(0000) knlGS:0000000000000000
20554 [ 309.858749] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
20555 [ 309.864519] CR2: 00007f9fca701000 CR3: 00000001565e2005 CR4: 0000000008f70ef0
20556 [ 309.871678] PKRU: 55555558
20557 [ 309.874403] Call Trace:
20558 [ 309.876866] <TASK>
20559 [ 309.878988] sg_alloc_table_from_pages_segment+0x60/0x100
20560 [ 309.884415] ? ttm_resource_manager_usage+0x36/0x60 [ttm]
20561 [ 309.889845] ? xe_tt_map_sg+0x7d/0xd0 [xe]
20562 [ 309.894045] xe_tt_map_sg+0x7d/0xd0 [xe]
20563 [ 309.898037] xe_bo_move+0x927/0xaa0 [xe]
20564 [ 309.902029] ttm_bo_handle_move_mem+0xba/0x170 [ttm]
20565 [ 309.907022] ttm_bo_validate+0xbe/0x190 [ttm]
20566 [ 309.911405] xe_bo_validate+0x9a/0x120 [xe]
20567 [ 309.915663] xe_gpuvm_validate+0xd9/0x140 [xe]
20568 [ 309.920206] drm_gpuvm_validate+0x2f0/0x5b0 [drm_gpuvm]
20569 [ 309.925459] ? drm_exec_lock_obj+0x63/0x210 [drm_exec]
20570 [ 309.930627] xe_vm_validate_rebind+0x46/0xb0 [xe]
20571 [ 309.935428] xe_exec_fn+0x20/0x40 [xe]
20572 [ 309.939249] drm_gpuvm_exec_lock+0x78/0xc0 [drm_gpuvm]
20573 [ 309.944410] xe_validation_exec_lock+0x5a/0xa0 [xe]
20574 [ 309.949385] xe_exec_ioctl+0x806/0xc30 [xe]
20575 [ 309.953639] ? ttwu_queue_wakelist+0xd9/0xf0
20576 [ 309.957935] ? __pfx_xe_exec_fn+0x10/0x10 [xe]
20577 [ 309.962449] ? __wake_up_common+0x73/0xa0
20578 [ 309.966482] ? __pfx_xe_exec_ioctl+0x10/0x10 [xe]
20579 [ 309.971263] drm_ioctl_kernel+0xa3/0x100
20580 [ 309.975209] drm_ioctl+0x213/0x440
20581 [ 309.978637] ? __pfx_xe_exec_ioctl+0x10/0x10 [xe]
20582 [ 309.983415] xe_drm_ioctl+0x67/0xd0 [xe]
20583 [ 309.987408] __x64_sys_ioctl+0x7f/0xd0
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the Linux kernel's Direct Rendering Manager Translation Table Manager subsystem represents a critical state management flaw within the memory backup and restore mechanisms used by GPU drivers, specifically impacting Intel Xe graphics implementations. The core issue resides in the ttm_pool_restore_and_alloc function, which is responsible for restoring previously backed-up buffer objects from system memory back into their original locations or alternative memory types. During this process, the kernel utilizes shmem handles to manage page mappings efficiently. While the restoration logic successfully commits the restore operation via ttm_pool_restore_commit, it fails to clear the tt->restore flag and properly release associated resources upon completion. This oversight creates a discrepancy between the perceived state of the translation table entry and its actual internal configuration. Specifically, while the system believes the restore has completed cleanly, shmem handles remain installed in the page structures without being correctly unlinked or freed from the restoration context.
This inconsistency leads to severe operational instability when subsequent memory management operations attempt to interact with these corrupted entries. The primary symptom is a kernel panic triggered during scatter-gather table allocation attempts. When the system tries to map pages for buffer object movement, it invokes sg_alloc_append_table_from_pages, which expects valid and consistent page arrays. Because the previous restore operation left residual shmem handles in tt->pages without clearing the restoration state flag, subsequent validation routines encounter invalid or double-mapped resources. The call trace indicates that this failure occurs deep within the execution path of GPU command submission, specifically during xe_bo_validate and ttm_bo_handle_move_mem operations. These functions are critical for ensuring buffer objects reside in appropriate memory regions before being submitted to the hardware for processing commands via ioctl interfaces like xe_exec_ioctl.
The impact of this vulnerability is primarily a denial of service against the graphics subsystem and potentially the entire system due to an unhandled kernel exception. An attacker or even normal application workload patterns that trigger frequent backup-restore cycles, such as heavy 3D rendering, video decoding with memory migration, or virtual machine GPU passthrough scenarios, can exploit this state corruption. The resulting crash halts all graphical output and may require a hard reboot to recover service continuity. From a security perspective, while the immediate effect is stability-related, kernel crashes in privileged subsystems often present opportunities for further exploitation if an attacker can control the memory layout or timing of subsequent allocations before the system reboots. However, the primary classification remains as a resource management error leading to system instability rather than direct privilege escalation.
From a technical standards perspective, this vulnerability aligns with CWE-401, which describes missing release of memory after successful allocation, and more specifically CWE-362, concurrent execution using shared resources with insufficient synchronization or state tracking. The failure to clear the tt->restore flag constitutes a logic error where the object's lifecycle state is not properly synchronized with its resource usage. In terms of MITRE ATT&CK mapping for Linux systems, this falls under T1499 Endpoint Denial of Service, as it allows an unprivileged user or local process to crash the kernel through legitimate API calls that trigger the flawed code path. The vulnerability highlights a common class of bugs in complex driver stacks where error handling paths and success paths diverge incorrectly regarding resource cleanup duties.
Mitigation for this issue requires applying the upstream Linux kernel patch that modifies ttm_pool_restore_and_alloc to explicitly free and drop tt->restore upon successful completion of the restore process. System administrators should ensure their distributions are updated with kernels containing this fix, which has been integrated into recent stable releases. For environments where immediate patching is not feasible, limiting GPU-intensive workloads or disabling specific memory migration features in the Xe driver configuration may reduce the likelihood of triggering the condition until a full system update can be performed. Developers integrating custom DRM drivers should audit their own backup and restore implementations to ensure that state flags are cleared consistently with resource deallocation to prevent similar logic errors from manifesting in other subsystems.