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

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

On 27/11/2025 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>  

dim (rightly) complains about these tags. Co-developed-by is a tricky
one because it needs to be paired with an identical Signed-off-by (see
the docs[1]) because it states that Boris has written some of the code.

[1]
https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by

>>
>> 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.

I feel a bit like the tag police, but it's good to get them right so the
backports go smoothly (in this case we shouldn't need a backport and it
can go into drm-misc-next-fixes).

Thanks,
Steve

>>
>> 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;  
>>
>