linux-kernel - Re: [PATCH] drm/panthor: fix for dma-fence safe access rules

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Message-ID: <34d18685-d07f-4e13-b5ca-d4d75d33d6f6@igalia.com>
Date: Fri, 5 Dec 2025 12:50:18 +0000
From: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>
To: Boris Brezillon <boris.brezillon@...labora.com>,
 Chia-I Wu <olvaffe@...il.com>
Cc: Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Grant Likely <grant.likely@...aro.org>, Heiko Stuebner <heiko@...ech.de>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/panthor: fix for dma-fence safe access rules


On 05/12/2025 12:46, Boris Brezillon wrote:
> On Thu, 4 Dec 2025 09:42:37 -0800
> Chia-I Wu <olvaffe@...il.com> wrote:
> 
>> On Thu, Dec 4, 2025 at 1:27 AM Tvrtko Ursulin <tvrtko.ursulin@...lia.com> wrote:
>>>
>>>
>>> On 04/12/2025 01:50, Chia-I Wu wrote:
>>>> Commit 506aa8b02a8d6 ("dma-fence: Add safe access helpers and document
>>>> the rules") details the dma-fence safe access rules. The most common
>>>> culprit is that drm_sched_fence_get_timeline_name may race with
>>>> group_free_queue.
>>>>
>>>> Fixes: d2624d90a0b77 ("drm/panthor: assign unique names to queues")
>>>> Signed-off-by: Chia-I Wu <olvaffe@...il.com>
>>>> ---
>>>>    drivers/gpu/drm/panthor/panthor_sched.c | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
>>>> index 33b9ef537e359..a8b1347e4da71 100644
>>>> --- a/drivers/gpu/drm/panthor/panthor_sched.c
>>>> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
>>>> @@ -23,6 +23,7 @@
>>>>    #include <linux/module.h>
>>>>    #include <linux/platform_device.h>
>>>>    #include <linux/pm_runtime.h>
>>>> +#include <linux/rcupdate.h>
>>>>
>>>>    #include "panthor_devfreq.h"
>>>>    #include "panthor_device.h"
>>>> @@ -923,6 +924,9 @@ static void group_release_work(struct work_struct *work)
>>>>                                                   release_work);
>>>>        u32 i;
>>>>
>>>> +     /* dma-fences may still be accessing group->queues under rcu lock. */
>>>> +     synchronize_rcu();
>>>> +
>>>>        for (i = 0; i < group->queue_count; i++)
>>>>                group_free_queue(group, group->queues[i]);
>>>>   
>>>
>>> This handles the shared queue->fence_ctx.lock as well (which is also
>>> unsafe until Christian lands the inline lock, etc patch series) so it
>>> looks good to me as well.
>> Yeah, I will send v2 to drop the misleading "Fixes:" tag.
>>
>> FWIW, the UAF I saw was from accessing the string returned by
>>
>> static const char *drm_sched_fence_get_timeline_name(struct dma_fence *f)
>> {
>>          struct drm_sched_fence *fence = to_drm_sched_fence(f);
>>          return (const char *)fence->sched->name;
>> }
> 
> IIRC, the only place calling this callback is some debugsfs knob
> dumping fences attached to dma_buf resvs, and we're not supposed to
> expose our driver fences to the outside world (we use the
> drm_sched_fence proxy for that), so I'm curious where the access was
> coming from.

Via the sync_file uapi.

For reference here is a reproducer for xe:
https://patchwork.freedesktop.org/patch/642709/?series=146211&rev=2

Regards,

Tvrtko


> 
>>
>> I thought it was "name" and added the "Fixes:" tag. But actually
>> "sched" was also freed by group_release_work.
>>
>>>
>>> Just to mention an alternative could be to simply switch release_work to
>>> INIT_RCU_WORK/queue_rcu_work, but I am not sure if that has an advantage.
>>>
>>> Regards,
>>>
>>> Tvrtko
>>>   
>