[PATCH v4 2/5] drm/xe/svm: Fix a potential bo UAF

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

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.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/xe/xe_svm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
index 52613dd8573a..c7424c824a14 100644
--- a/drivers/gpu/drm/xe/xe_svm.c
+++ b/drivers/gpu/drm/xe/xe_svm.c
@@ -702,11 +702,14 @@ 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);
-- 
2.48.1