linux-kernel - Re: [PATCH v2] fbdev: Use helper to get fb

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Message-ID: <d47a3cab-4f21-3b8b-2834-030663677070@suse.de>
Date:   Wed, 4 May 2022 13:08:43 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Junxiao Chang <junxiao.chang@...el.com>,
        dri-devel@...ts.freedesktop.org, Maxime Ripard <maxime@...no.tech>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v2] fbdev: Use helper to get fb_info in all file
 operations

Hi

Am 04.05.22 um 12:55 schrieb Daniel Vetter:
> On Wed, May 04, 2022 at 11:28:07AM +0200, Javier Martinez Canillas wrote:
>> Hello Daniel,
>>
>> On 5/4/22 11:02, Daniel Vetter wrote:
>>> On Tue, May 03, 2022 at 10:19:34PM +0200, Javier Martinez Canillas wrote:
>>>> A reference to the framebuffer device struct fb_info is stored in the file
>>>> private data, but this reference could no longer be valid and must not be
>>>> accessed directly. Instead, the file_fb_info() accessor function must be
>>>> used since it does sanity checking to make sure that the fb_info is valid.
>>>>
>>>> This can happen for example if the registered framebuffer device is for a
>>>> driver that just uses a framebuffer provided by the system firmware. In
>>>> that case, the fbdev core would unregister the framebuffer device when a
>>>> real video driver is probed and ask to remove conflicting framebuffers.
>>>>
>>>> Most fbdev file operations already use the helper to get the fb_info but
>>>> get_fb_unmapped_area() and fb_deferred_io_fsync() don't. Fix those two.
>>>>
>>>> Since fb_deferred_io_fsync() is not in fbmem.o, the helper has to be
>>>> exported. Rename it and add a fb_ prefix to denote that is public now.
>>>>
>>>> Reported-by: Junxiao Chang <junxiao.chang@...el.com>
>>>> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
>>>
>>> Note that fb_file_info is hilariously racy since there's nothing
>>> preventing a concurrenct framebuffer_unregister. Or at least I'm not
>>> seeing anything. See cf4a3ae4ef33 ("fbdev: lock_fb_info cannot fail") for
>>> context, maybe reference that commit here in your patch.
>>>
>>> Either way this doesn't really make anything worse, so
>>> Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>
>>>
>>
>> Yes, I noticed is racy but at least checking this makes less likely to
>> occur. And thanks, I'll reference that commit in the description of v3.
>>
>> BTW, I also noticed that the same race that happens with open(),read(),
>> close(), etc happens with the VM operations:
>>
>> int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
>> {
>> ...
>> 	vma->vm_private_data = info;
>> ...
>> }
>>
>> static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>> {
>> ...
>> 	struct fb_info *info = vmf->vma->vm_private_data;
>> ...
>> }
>>
>> static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
>> {
>> ...
>> 	struct fb_info *info = vmf->vma->vm_private_data;
>> ...
>> }
>>
>> So something similar to fb_file_fb_info() is needed to check if
>> the vm_private_data is still valid. I guess that could be done
>> by using the vmf->vma->vm_file and attempting the same trick that
>> fb_file_fb_info() does ?
> 
> Yeah should work, except if the ptes are set up already there's kinda not
> much that this will prevent. We'd need to tear down mappings and SIGBUS or
> alternatively have something else in place there so userspace doesn't blow
> up in funny ways (which is what we're doing on the drm side, or at least
> trying to).
> 
> I'm also not sure how much we should care, since ideally for drm drivers
> this is all taken care of by drm_dev_enter in the right places. It does
> mean though that fbdev mmap either needs to have it's own memory or be
> fully redirected to the drm gem mmap.
> 
> And then we can afford to just not care to fix fbdev itself.

While the problem has been there ever since, the bug didn't happen until 
we fixed hot-unplugging for fbdev. Not doing anything is probably not 
the right thing.

Best regards
Thomas

> -Daniel

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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