Re: [PATCH v2 2/2] drm/xe/queue: move xa_alloc to prevent UAF

[Matthew Auld <matthew.auld@intel.com>]

On 25/09/2024 11:44, Nirmoy Das wrote:
> 
> On 9/25/2024 11:51 AM, Matthew Auld wrote:
>> On 25/09/2024 10:33, Nirmoy Das wrote:
>>>
>>> On 9/25/2024 9:14 AM, Matthew Auld wrote:
>>>> Evil user can guess the next id of the queue before the ioctl completes
>>>> and then call queue destroy ioctl to trigger UAF since create ioctl is
>>>> still referencing the same queue. Move the xa_alloc all the way to the end
>>>> to prevent this.
>>>
>>> The commit message doesn't match the diff, xa_alloc already happening at the end here.
>>
>> It's not at the end. It is dereferencing the q to set xef after the xa_alloc, but that needs to happen before it is visible to userspace.
> 
> 
> Ah I see, that make sense.
> 
> I think now  we have to undo xe_file_get() if xe_alloc fails.

That is already handled by the queue put().

> 
> 
> Regards,
> 
> Nirmoy
> 
>>
>>>
>>>> v2:
>>>>    - Rebase
>>>>
>>>> Fixes: 2149ded63079 ("drm/xe: Fix use after free when client stats are captured")
>>>> Signed-off-by: Matthew Auld<matthew.auld@intel.com>
>>>> Cc: Matthew Brost<matthew.brost@intel.com>
>>>> ---
>>>>    drivers/gpu/drm/xe/xe_exec_queue.c | 4 +++-
>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/xe/xe_exec_queue.c b/drivers/gpu/drm/xe/xe_exec_queue.c
>>>> index 7743ebdcbf4b..d098d2dd1b2d 100644
>>>> --- a/drivers/gpu/drm/xe/xe_exec_queue.c
>>>> +++ b/drivers/gpu/drm/xe/xe_exec_queue.c
>>>> @@ -635,12 +635,14 @@ int xe_exec_queue_create_ioctl(struct drm_device *dev, void *data,
>>>>            }
>>>>        }
>>>>    +    q->xef = xe_file_get(xef);
>>>> +
>>>> +    /* user id alloc must always be last in ioctl to prevent UAF */
>>>>        err = xa_alloc(&xef->exec_queue.xa, &id, q, xa_limit_32b, GFP_KERNEL);
>>>>        if (err)
>>>>            goto kill_exec_queue;
>>>>          args->exec_queue_id = id;
>>>> -    q->xef = xe_file_get(xef);
>>>>          return 0;
>>>>