[v18,22/26] drm/shmem-helper: Don't free refcounted GEM

  Don't free refcounted shmem object to prevent use-after-free bug that
is worse than a memory leak.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
  

On Mon, 30 Oct 2023 02:02:01 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> Don't free refcounted shmem object to prevent use-after-free bug that
> is worse than a memory leak.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 6dd087f19ea3..4253c367dc07 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  	if (obj->import_attach)
>  		drm_prime_gem_destroy(obj, shmem->sgt);
>  
> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
> +	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
> +		return;

I guess you're worried about ->sgt being referenced by the driver after
the GEM is destroyed. If we assume drivers don't cache the sgt and
always call get_pages_sgt() when they need it that shouldn't be an
issue. What we really don't want to release is the pages themselves,
but the GPU MMU might still have active mappings pointing to these
pages.

In any case, I'm not against leaking the GEM object when any of these
counters are not zero, but can we at least have a comment in the
code explaining why we're doing that, so people don't have to go look
at the git history to figure it out.

>  
>  	drm_gem_object_release(obj);
>  	kfree(shmem);
  

On 11/13/23 12:54, Boris Brezillon wrote:
> On Mon, 30 Oct 2023 02:02:01 +0300
> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> 
>> Don't free refcounted shmem object to prevent use-after-free bug that
>> is worse than a memory leak.
>>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index 6dd087f19ea3..4253c367dc07 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>  	if (obj->import_attach)
>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>>  
>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
>> +	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
>> +		return;
> 
> I guess you're worried about ->sgt being referenced by the driver after
> the GEM is destroyed. If we assume drivers don't cache the sgt and
> always call get_pages_sgt() when they need it that shouldn't be an
> issue. What we really don't want to release is the pages themselves,
> but the GPU MMU might still have active mappings pointing to these
> pages.
> 
> In any case, I'm not against leaking the GEM object when any of these
> counters are not zero, but can we at least have a comment in the
> code explaining why we're doing that, so people don't have to go look
> at the git history to figure it out.

This patch is a minor improvement, it doesn't address any specific
issue. This should be a common pattern in kernel. If you're giving a
warning and know about the inevitable catastrophe, then avoid it if you can.

Actually, there are other similar cases in drm-shmem that can be improved.
  

On Thu, 23 Nov 2023 01:30:24 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 11/13/23 12:54, Boris Brezillon wrote:
> > On Mon, 30 Oct 2023 02:02:01 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> >> Don't free refcounted shmem object to prevent use-after-free bug that
> >> is worse than a memory leak.
> >>
> >> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> >> ---
> >>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
> >>  1 file changed, 4 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index 6dd087f19ea3..4253c367dc07 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>  	if (obj->import_attach)
> >>  		drm_prime_gem_destroy(obj, shmem->sgt);
> >>  
> >> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
> >> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
> >> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
> >> +	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
> >> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
> >> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
> >> +		return;  
> > 
> > I guess you're worried about ->sgt being referenced by the driver after
> > the GEM is destroyed. If we assume drivers don't cache the sgt and
> > always call get_pages_sgt() when they need it that shouldn't be an
> > issue. What we really don't want to release is the pages themselves,
> > but the GPU MMU might still have active mappings pointing to these
> > pages.
> > 
> > In any case, I'm not against leaking the GEM object when any of these
> > counters are not zero, but can we at least have a comment in the
> > code explaining why we're doing that, so people don't have to go look
> > at the git history to figure it out.  
> 
> This patch is a minor improvement, it doesn't address any specific
> issue. This should be a common pattern in kernel. If you're giving a
> warning and know about the inevitable catastrophe, then avoid it if you can.

Sure, I'm just asking that we add a comment to explain why we leak
memory here. Is that too much to ask?
  

On 11/23/23 12:08, Boris Brezillon wrote:
> On Thu, 23 Nov 2023 01:30:24 +0300
> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> 
>> On 11/13/23 12:54, Boris Brezillon wrote:
>>> On Mon, 30 Oct 2023 02:02:01 +0300
>>> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
>>>   
>>>> Don't free refcounted shmem object to prevent use-after-free bug that
>>>> is worse than a memory leak.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>>>> ---
>>>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> index 6dd087f19ea3..4253c367dc07 100644
>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>>>  	if (obj->import_attach)
>>>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>>>>  
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
>>>> +	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
>>>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
>>>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
>>>> +		return;  
>>>
>>> I guess you're worried about ->sgt being referenced by the driver after
>>> the GEM is destroyed. If we assume drivers don't cache the sgt and
>>> always call get_pages_sgt() when they need it that shouldn't be an
>>> issue. What we really don't want to release is the pages themselves,
>>> but the GPU MMU might still have active mappings pointing to these
>>> pages.
>>>
>>> In any case, I'm not against leaking the GEM object when any of these
>>> counters are not zero, but can we at least have a comment in the
>>> code explaining why we're doing that, so people don't have to go look
>>> at the git history to figure it out.  
>>
>> This patch is a minor improvement, it doesn't address any specific
>> issue. This should be a common pattern in kernel. If you're giving a
>> warning and know about the inevitable catastrophe, then avoid it if you can.
> 
> Sure, I'm just asking that we add a comment to explain why we leak
> memory here. Is that too much to ask?

Will add the comment. The reason why I added this patch was unrelated to
the sgt, that's what I'm talking about.