drm/xe/svm: Fix a potential bo UAF

If drm_gpusvm_migrate_to_devmem() succeeds, if a cpu access happens to the
range the bo may be freed before xe_bo_unlock(), causing a UAF.

Since the reference is transferred, use xe_svm_devmem_release() to
release the reference on drm_gpusvm_migrate_to_devmem() failure,
and hold a local reference to protect the UAF.

Fixes: 2f118c9 ("drm/xe: Add SVM VRAM migration")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250326080551.40201-3-thomas.hellstrom@linux.intel.com
(cherry picked from commit c9db07c)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Author: Thomas Hellström

drivers/gpu/drm/xe/xe_svm.c

@@ -696,11 +696,14 @@ static int xe_svm_alloc_vram(struct xe_vm *vm, struct xe_tile *tile,
 	list_for_each_entry(block, blocks, link)
 		block->private = vr;
 
+	xe_bo_get(bo);
 	err = drm_gpusvm_migrate_to_devmem(&vm->svm.gpusvm, &range->base,
 					   &bo->devmem_allocation, ctx);
-	xe_bo_unlock(bo);
 	if (err)
-		xe_bo_put(bo);	/* Creation ref */
+		xe_svm_devmem_release(&bo->devmem_allocation);
+
+	xe_bo_unlock(bo);
+	xe_bo_put(bo);
 
 unlock:
 	mmap_read_unlock(mm);