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Message-ID: <c137394d-bd25-4e7d-aaba-32beffb12ec3@arm.com>
Date: Thu, 27 Nov 2025 16:39:39 +0000
From: Steven Price <steven.price@....com>
To: Akash Goel <akash.goel@....com>,
Boris Brezillon <boris.brezillon@...labora.com>
Cc: liviu.dudau@....com, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org, maarten.lankhorst@...ux.intel.com,
mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch,
nd@....com
Subject: Re: [PATCH] drm/panthor: Prevent potential UAF in group creation
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@....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@...labora.com>
>>>> Signed-off-by: Akash Goel <akash.goel@....com>
>>>
>>> Reviewed-by: Steven Price <steven.price@....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;
>>>
>>
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