[PATCH] drm/xe/vm: nuke PTs only after unlinking contested VMAs

[Matthew Auld <matthew.auld@intel.com>]

In xe_vm_close_and_put(), external-BO VMAs are queued on the contested
list for deferred destruction via xe_vma_destroy_unlocked(). However,
xe_vm_pt_destroy() was previously invoked before processing contested
VMAs, destroying vm->pt_root while those VMAs were still linked to their
respective buffer objects (vm_bo->list.gpuva).

If a concurrent thread evicts one of those shared buffer objects,
xe_bo_trigger_rebind() holding only bo->resv walks the BO's VMAs and, in
fault mode, calls xe_vm_invalidate_vma() -> xe_pt_zap_ptes(). Because
vm->pt_root[tile->id] is already NULL, dereferencing pt->level causes a
NULL ptr deref.

Fix this by deferring xe_vm_free_scratch() and xe_vm_pt_destroy() until
after all contested VMAs have been unlinked and destroyed.

User is reporting hitting a NULL ptr deref in xe_pt_zap_ptes(), which
could be explained by this race.

Assisted-by: LLM
Fixes: b06d47be7c83 ("drm/xe: Port Xe to GPUVA")
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/9290
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.12+
---
 drivers/gpu/drm/xe/xe_vm.c | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index 17dc4debe7c1..390da884c727 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -1979,21 +1979,13 @@ void xe_vm_close_and_put(struct xe_vm *vm)
 		vma->gpuva.flags |= XE_VMA_DESTROYED;
 	}
 
-	/*
-	 * All vm operations will add shared fences to resv.
-	 * The only exception is eviction for a shared object,
-	 * but even so, the unbind when evicted would still
-	 * install a fence to resv. Hence it's safe to
-	 * destroy the pagetables immediately.
-	 */
-	xe_vm_free_scratch(vm);
-	xe_vm_pt_destroy(vm);
 	xe_vm_unlock(vm);
 
 	/*
-	 * VM is now dead, cannot re-add nodes to vm->vmas if it's NULL
-	 * Since we hold a refcount to the bo, we can remove and free
-	 * the members safely without locking.
+	 * Unlink and destroy all contested external-BO VMAs before destroying
+	 * the page tables. Otherwise, concurrent eviction holding only bo->resv
+	 * can walk the BO's VMAs and attempt to invalidate/zap page tables that
+	 * have already been freed.
 	 */
 	list_for_each_entry_safe(vma, next_vma, &contested,
 				 combined_links.destroy) {
@@ -2001,6 +1993,11 @@ void xe_vm_close_and_put(struct xe_vm *vm)
 		xe_vma_destroy_unlocked(vma);
 	}
 
+	xe_vm_lock(vm, false);
+	xe_vm_free_scratch(vm);
+	xe_vm_pt_destroy(vm);
+	xe_vm_unlock(vm);
+
 	xe_svm_fini(vm);
 
 	up_write(&vm->lock);
-- 
2.55.0