Re: [PATCH 1/9] dma-buf: add dma_fence_was_initialized function v2

[Jiri Slaby <jirislaby@kernel.org>]

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)) {
>   		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. And why it 
did not fail before?
  

 >         } else { 

 >                 qxl_release_free_list(release); 

 >                 kfree(release); 

 >         } 

 >         atomic_dec(&qdev->release_count); 

 > }

thanks,
-- 
js
suse labs