On Mon, 2026-07-13 at 10:58 +0200, Jiri Slaby wrote:

Hi,

On 20. 01. 26, 11:54, Christian König wrote:

...

Some driver use fence->ops to test if a fence was initialized or not. The problem is that this utilizes internal behavior of the dma_fence implementation.

So better abstract that into a function.

v2: use a flag instead of testing fence->ops, rename the function, move      to the beginning of the patch set.

...

...

--- a/drivers/gpu/drm/qxl/qxl_release.c +++ b/drivers/gpu/drm/qxl/qxl_release.c @@ -146,7 +146,7 @@ qxl_release_free(struct qxl_device *qdev,    idr_remove(&qdev->release_idr, release->id);    spin_unlock(&qdev->release_idr_lock);

• if (release->base.ops) {

• if (dma_fence_was_initialized(&release->base)) {

Could you verify the cause with sth like

if (release->base.ops && dma_fence_was_initialized(…)) {

?

...

WARN_ON(list_empty(&release->bos));    qxl_release_free_list(release);

This likely breaks qxl: https://bugzilla.suse.com/show_bug.cgi?id=1271081

> refcount_t: underflow; use-after-free.  > WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x59/0x90, CPU#0: kworker/0:0/1534  > Modules linked in: af_packet nft_fib_inet ...  > CPU: 0 UID: 0 PID: 1534 Comm: kworker/0:0 Not tainted 7.1.3-1-default #1 PREEMPT(full) openSUSE Tumbleweed b041a6527f6e58424f4cd3de0fade8d408b378fd  > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-2-gc13ff2cd-prebuilt.qemu.org 04/01/2014  > Workqueue: events qxl_gc_work [qxl]  > RIP: 0010:refcount_warn_saturate+0x59/0x90  > Code: 44 48 8d 3d a9 72 b7 01 67 48 0f b9 3a c3 cc cc cc cc 48 8d 3d a8 72 b7 01 67 48 0f b9 3a c3 cc cc cc cc 48 8d 3d a7 72 b7 01 <67> 48 0f b9 3a c3 cc cc cc cc 48 8d 3d a6 72 b7 01 67 48 0f b9 3a  > RSP: 0018:ffffce09019efe00 EFLAGS: 00010246  > RAX: 0000000000000000 RBX: ffff8b5a56f819c0 RCX: 0000000000000017  > RDX: 0000000000011696 RSI: 0000000000000003 RDI: ffffffffbc953b70  > RBP: ffff8b5a4a314000 R08: 0000000000000001 R09: ffffffffc07d488a  > R10: fffffa6fc4787d80 R11: ffff8b5a40044000 R12: 0000000000000004  > R13: 0000000000000003 R14: ffffce09003f1434 R15: ffff8b5a410c4640  > FS:  0000000000000000(0000) GS:ffff8b5afe954000(0000) knlGS:0000000000000000  > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033  > CR2: 00007ff463d65000 CR3: 000000012b476004 CR4: 0000000000772ef0  > PKRU: 55555554  > Call Trace:  >  <TASK>  >  qxl_release_free+0xee/0xf0 [qxl d93e9381353e619799d56790f5f8dda6cce491f6]  >  qxl_garbage_collect+0xd1/0x1b0 [qxl d93e9381353e619799d56790f5f8dda6cce491f6]  >  process_one_work+0x19e/0x3a0  > ...

The function does now:

> void  > qxl_release_free(struct qxl_device *qdev,  >                  struct qxl_release *release)  > {  > ...  >         if (dma_fence_was_initialized(&release->base)) {  >                 WARN_ON(list_empty(&release->bos));

>                 qxl_release_free_list(release);

>

>                 dma_fence_signal(&release->base);

>                 dma_fence_put(&release->base);

refcount_dec_and_test() in kref_put() in this ^^ crashes. Apparently, the reference count was not increased anywhere. QXL does not call dma_fence_get() -- who is supposed to increase the refcount.

dma_fence_init() initializes the refcount to 1. So since there is only one place with dma_fence_put(), it probably aims at countering that.

And why it did not fail before?

My guess would be that base->ops was used as a boolean to see whether qxl_release contains a valid fence. It seems qxl_release_free() is intended as some sort of universal function; it is called at many places. So I would suppose it is sometimes being called without the fence being initialized.

Should most certainly have been documented. Anyways, verifying the issue would probably be cool

Cheers, P.

>         } else {

>                 qxl_release_free_list(release);

>                 kfree(release);

>         }

>         atomic_dec(&qdev->release_count);

> }

thanks,