Re: [PATCH] drm/panthor: Prevent potential UAF in group creation

[Steven Price <steven.price@arm.com>]

On 27/11/2025 16:32, Akash Goel wrote:
> Hi Steve, Boris
> 
> On 11/27/25 16:08, Boris Brezillon wrote:
>> On Thu, 27 Nov 2025 16:02:15 +0000
>> Steven Price <steven.price@arm.com> wrote:
>>
>>> On 27/11/2025 08:12, Akash Goel wrote:
>>>> This commit prevents the possibility of a use after free issue in the
>>>> GROUP_CREATE ioctl function, which arose as pointer to the group is
>>>> accessed in that ioctl function after storing it in the Xarray.
>>>> A malicious userspace can second guess the handle of a group and try
>>>> to call GROUP_DESTROY ioctl from another thread around the same time
>>>> as GROUP_CREATE ioctl.
>>>>
>>>> To prevent the use after free exploit, this commit uses a mark on an
>>>> entry of group pool Xarray which is added just before returning from
>>>> the GROUP_CREATE ioctl function. The mark is checked for all ioctls
>>>> that specify the group handle and so userspace won't be abe to delete
>>>> a group that isn't marked yet.
>>>>
>>>> Co-developed-by: Boris Brezillon <boris.brezillon@collabora.com>
>>>> Signed-off-by: Akash Goel <akash.goel@arm.com>
>>>
>>> Reviewed-by: Steven Price <steven.price@arm.com>
>>>
>>> I *think* this should have a...
>>>
>>> Fixes: d2624d90a0b7 ("drm/panthor: assign unique names to queues")
>>>
>>> ... as I don't believe it was a problem before the rearrangement that
>>> happened there.
>>
>> Oh, yeah, I didn't notice the commit was missing a Fixes tag, and
>> you're correct about the offending commit.
>>
> 
> Sorry for not adding the Fixes tag.
> 
> 
> I think the problem has been present since the beginning and the Fixes
> tag should be
> 
> Fixes: de85488138247 ("drm/panthor: Add the scheduler logical block")
> 
> 
> Initially the code was like this,
> 
> 
>     ret = xa_alloc(&gpool->xa, &gid, group, XA_LIMIT(1,
> MAX_GROUPS_PER_POOL), GFP_KERNEL);
>     if (ret)
>         goto err_put_group;
> 
>     mutex_lock(&sched->reset.lock);
>     if (atomic_read(&sched->reset.in_progress)) {
>         panthor_group_stop(group);
>     } else {
>         mutex_lock(&sched->lock);
>         list_add_tail(&group->run_node,
>                   &sched->groups.idle[group->priority]);
>         mutex_unlock(&sched->lock);
>     }
>     mutex_unlock(&sched->reset.lock);
> 
>     return gid;
> 
> If the GROUP_CREATE ioctl thread somehow gets preempted immediately
> after xa_alloc(), then another thread might succeed in freeing the group
> through GROUP_DESTROY ioctl.
> 
> Initially it would have been very difficult to trigger the UAF, but
> d2624d90a0b7 ("drm/panthor: assign unique names to queues") would have
> made the code more susceptible to UAF.
> 
> Please kindly correct me if I interpreted things incorrectly.

No, looking at this again, I think you're correct - I saw the locks and
didn't think through that they don't actually protect us against the
racing destroy. I'm not sure why that code didn't have the xa_alloc() as
the final call.

> Will accordingly send a v2.

Thanks!

Steve

> Best regards
> Akash
> 
> 
>>>
>>> Thanks,
>>> Steve
>>>
>>>> ---
>>>>   drivers/gpu/drm/panthor/panthor_sched.c | 19 +++++++++++++++----
>>>>   1 file changed, 15 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/
>>>> drm/panthor/panthor_sched.c
>>>> index b834123a6560..a6b8024e1a3c 100644
>>>> --- a/drivers/gpu/drm/panthor/panthor_sched.c
>>>> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
>>>> @@ -779,6 +779,12 @@ struct panthor_job_profiling_data {
>>>>    */
>>>>   #define MAX_GROUPS_PER_POOL 128
>>>>   +/*
>>>> + * Mark added on an entry of group pool Xarray to identify if the
>>>> group has
>>>> + * been fully initialized and can be accessed elsewhere in the
>>>> driver code.
>>>> + */
>>>> +#define GROUP_REGISTERED XA_MARK_1
>>>> +
>>>>   /**
>>>>    * struct panthor_group_pool - Group pool
>>>>    *
>>>> @@ -3007,7 +3013,7 @@ void
>>>> panthor_fdinfo_gather_group_samples(struct panthor_file *pfile)
>>>>           return;
>>>>         xa_lock(&gpool->xa);
>>>> -    xa_for_each(&gpool->xa, i, group) {
>>>> +    xa_for_each_marked(&gpool->xa, i, group, GROUP_REGISTERED) {
>>>>           guard(spinlock)(&group->fdinfo.lock);
>>>>           pfile->stats.cycles += group->fdinfo.data.cycles;
>>>>           pfile->stats.time += group->fdinfo.data.time;
>>>> @@ -3727,6 +3733,8 @@ int panthor_group_create(struct panthor_file
>>>> *pfile,
>>>>         group_init_task_info(group);
>>>>   +    xa_set_mark(&gpool->xa, gid, GROUP_REGISTERED);
>>>> +
>>>>       return gid;
>>>>     err_erase_gid:
>>>> @@ -3744,6 +3752,9 @@ int panthor_group_destroy(struct panthor_file
>>>> *pfile, u32 group_handle)
>>>>       struct panthor_scheduler *sched = ptdev->scheduler;
>>>>       struct panthor_group *group;
>>>>   +    if (!xa_get_mark(&gpool->xa, group_handle, GROUP_REGISTERED))
>>>> +        return -EINVAL;
>>>> +
>>>>       group = xa_erase(&gpool->xa, group_handle);
>>>>       if (!group)
>>>>           return -EINVAL;
>>>> @@ -3769,12 +3780,12 @@ int panthor_group_destroy(struct
>>>> panthor_file *pfile, u32 group_handle)
>>>>   }
>>>>     static struct panthor_group *group_from_handle(struct
>>>> panthor_group_pool *pool,
>>>> -                           u32 group_handle)
>>>> +                           unsigned long group_handle)
>>>>   {
>>>>       struct panthor_group *group;
>>>>         xa_lock(&pool->xa);
>>>> -    group = group_get(xa_load(&pool->xa, group_handle));
>>>> +    group = group_get(xa_find(&pool->xa, &group_handle,
>>>> group_handle, GROUP_REGISTERED));
>>>>       xa_unlock(&pool->xa);
>>>>         return group;
>>>> @@ -3861,7 +3872,7 @@ panthor_fdinfo_gather_group_mem_info(struct
>>>> panthor_file *pfile,
>>>>           return;
>>>>         xa_lock(&gpool->xa);
>>>> -    xa_for_each(&gpool->xa, i, group) {
>>>> +    xa_for_each_marked(&gpool->xa, i, group, GROUP_REGISTERED) {
>>>>           stats->resident += group->fdinfo.kbo_sizes;
>>>>           if (group->csg_id >= 0)
>>>>               stats->active += group->fdinfo.kbo_sizes;
>>>
>>