[PATCH v2 1/2] drm/xe/vm: move xa_alloc to prevent UAF

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

Evil user can guess the next id of the vm before the ioctl completes and
then call vm destroy ioctl to trigger UAF since create ioctl is still
referencing the same vm. Move the xa_alloc all the way to the end to
prevent this.

v2:
 - Rebase

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
---
 drivers/gpu/drm/xe/xe_vm.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index 31fe31db3fdc..ce9dca4d4e87 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -1765,10 +1765,6 @@ int xe_vm_create_ioctl(struct drm_device *dev, void *data,
 	if (IS_ERR(vm))
 		return PTR_ERR(vm);
 
-	err = xa_alloc(&xef->vm.xa, &id, vm, xa_limit_32b, GFP_KERNEL);
-	if (err)
-		goto err_close_and_put;
-
 	if (xe->info.has_asid) {
 		down_write(&xe->usm.lock);
 		err = xa_alloc_cyclic(&xe->usm.asid_to_vm, &asid, vm,
@@ -1776,12 +1772,11 @@ int xe_vm_create_ioctl(struct drm_device *dev, void *data,
 				      &xe->usm.next_asid, GFP_KERNEL);
 		up_write(&xe->usm.lock);
 		if (err < 0)
-			goto err_free_id;
+			goto err_close_and_put;
 
 		vm->usm.asid = asid;
 	}
 
-	args->vm_id = id;
 	vm->xef = xe_file_get(xef);
 
 	/* Record BO memory for VM pagetable created against client */
@@ -1794,10 +1789,15 @@ int xe_vm_create_ioctl(struct drm_device *dev, void *data,
 	args->reserved[0] = xe_bo_main_addr(vm->pt_root[0]->bo, XE_PAGE_SIZE);
 #endif
 
+	/* user id alloc must always be last in ioctl to prevent UAF */
+	err = xa_alloc(&xef->vm.xa, &id, vm, xa_limit_32b, GFP_KERNEL);
+	if (err)
+		goto err_close_and_put;
+
+	args->vm_id = id;
+
 	return 0;
 
-err_free_id:
-	xa_erase(&xef->vm.xa, id);
 err_close_and_put:
 	xe_vm_close_and_put(vm);
 
-- 
2.46.1