Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb_release()

[Javier Martinez Canillas]

On 5/10/22 09:19, Andrzej Hajda wrote:
> 
> 
> On 10.05.2022 00:42, Javier Martinez Canillas wrote:
>> On 5/10/22 00:22, Andrzej Hajda wrote:
>>
>> [snip]
>>
>>>>    static void drm_fbdev_fb_destroy(struct fb_info *info)
>>>>    {
>>>> +       if (info->cmap.len)
>>>> +               fb_dealloc_cmap(&info->cmap);
>>>> +
>>>>           drm_fbdev_release(info->par);
>>>> +       framebuffer_release(info);
>>> I would put drm_fbdev_release at the beginning - it cancels workers
>>> which could expect cmap to be still valid.
>>>
>> Indeed, you are correct again. [0] is the final version of the patch I've
>> but don't have an i915 test machine to give it a try. I'll test tomorrow
>> on my test systems to verify that it doesn't cause any regressions since
>> with other DRM drivers.
>>
>> I think that besides this patch, drivers shouldn't need to call to the
>> drm_fb_helper_fini() function directly. Since that would be called during
>> drm_fbdev_fb_destroy() anyways.
>>
>> We should probably remove that call in all drivers and make this helper
>> function static and just private to drm_fb_helper functions.
>>
>> Or am I missing something here ?
> 
> This is question for experts :)

Fair. I'm definitely not one of them :)

> I do not know what are user API/ABI expectations regarding removal of 
> fbdev driver, I wonder if they are documented somewhere :)

I don't know. At least I haven't found them.

> Apparently we have some process of 'zombification'  here - we need to 
> remove the driver without waiting for userspace closing framebuffer(???) 
> (to unbind ops-es and remove references to driver related things), but 
> we need to leave some structures to fool userspace, 'info' seems to be 
> one of them.

That's correct, yes. I think that any driver that provides a .mmap file
operation would have the same issue. But drivers keep an internal state
and just return -ENODEV or whatever on read/write/close after a removal.

The fbdev subsystem is different though since as you said it, the fbdev
core unconditionally calls to the driver .fb_release() callback with a
struct fb_info reference as argument.

I tried to prevent that with commit aafa025c76dc ("fbdev: Make fb_release()
return -ENODEV if fbdev was unregistered") but Daniel pointed out that
is was wrong since could leak memory allocated and was expected to be
freed on release.

That's why I instead fixed the issue in the fbdev drivers and just added
a warn on fb_release(), that is $SUBJECT.

> So I guess there should be something called on driver's _remove path, 
> and sth on destroy path.
>

That was my question actually, do we need something to be called in the
destroy path ? Since that could just be internal to the DRM fb helpers.

In other words, drivers should only care about setting a generic fbdev
by calling drm_fbdev_generic_setup(), and then do any HW cleanup in the
removal path, but let the fb helpers to handle the SW cleanup in destroy.
 
-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat