[PATCH] drm/amdkfd: Fix a __user pointer dereference in create_signal_event

[Yu, Lang]

[AMD Official Use Only]



>-----Original Message-----
>From: Lazar, Lijo <Lijo.Lazar at amd.com>
>Sent: Wednesday, October 13, 2021 4:07 PM
>To: Yu, Lang <Lang.Yu at amd.com>; amd-gfx at lists.freedesktop.org; Kuehling,
>Felix <Felix.Kuehling at amd.com>
>Cc: Deucher, Alexander <Alexander.Deucher at amd.com>; Huang, Ray
><Ray.Huang at amd.com>
>Subject: Re: [PATCH] drm/amdkfd: Fix a __user pointer dereference in
>create_signal_event
>
>
>
>On 10/13/2021 1:03 PM, Lang Yu wrote:
>> We should not dereference __user pointers directly.
>> https://yarchive.net/comp/linux/user_pointers.html
>>
>> Fixes: 482f07775cf5
>> ("drm/amdkfd: Simplify event ID and signal slot management")
>>
>> Signed-off-by: Lang Yu <lang.yu at amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdkfd/kfd_events.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_events.c
>> b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
>> index 3eea4edee355..74d3bdcfe341 100644
>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_events.c
>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
>> @@ -201,7 +201,7 @@ static int create_signal_event(struct file
>> *devkfd,
>>
>>   	p->signal_event_count++;
>>
>> -	ev->user_signal_address = &p->signal_page->user_address[ev-
>>event_id];
>
>This is interesting. I thought this wouldn't dereference.
>
>See here -
>
>https://en.cppreference.com/w/c/language/operator_member_access
>
>"If the operand is an array index expression, no action is taken other than the
>array-to-pointer conversion and the addition, so &a[N] is valid for an array of size
>N (obtaining a pointer one past the end is okay, dereferencing it is not, but
>dereference cancels out in this expression)"

Thanks for your clarification about this. I got it.

Regards,
Lang

>Thanks,
>Lijo
>
>
>> +	ev->user_signal_address = p->signal_page->user_address +
>> +ev->event_id;
>>   	pr_debug("Signal event number %zu created with id %d, address %p\n",
>>   			p->signal_event_count, ev->event_id,
>>   			ev->user_signal_address);
>>