[PATCH] drm/fbdev-generic: fix potential out-of-bounds access

[Thomas Zimmermann]

Hi

Am 13.04.23 um 22:01 schrieb Daniel Vetter:
> On Thu, Apr 13, 2023 at 09:20:23PM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 13.04.23 um 20:56 schrieb Daniel Vetter:
>> [...]
>>>
>>> This should switch the existing code over to using drm_framebuffer instead
>>> of fbdev:
>>>
>>>
>>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>>> index ef4eb8b12766..99ca69dd432f 100644
>>> --- a/drivers/gpu/drm/drm_fb_helper.c
>>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>>> @@ -647,22 +647,26 @@ static void drm_fb_helper_damage(struct drm_fb_helper *helper, u32 x, u32 y,
>>>    static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off, size_t len,
>>>    					       struct drm_rect *clip)
>>>    {
>>> +	struct drm_fb_helper *helper = info->par;
>>> +
>>>    	off_t end = off + len;
>>>    	u32 x1 = 0;
>>>    	u32 y1 = off / info->fix.line_length;
>>> -	u32 x2 = info->var.xres;
>>> -	u32 y2 = DIV_ROUND_UP(end, info->fix.line_length);
>>> +	u32 x2 = helper->fb->height;
>>> +	unsigned stride = helper->fb->pitches[0];
>>> +	u32 y2 = DIV_ROUND_UP(end, stride);
>>> +	int bpp = drm_format_info_bpp(helper->fb->format, 0);
>>
>> Please DONT do that. The code here is fbdev code and shouldn't bother about
>> DRM data structures. Actually, it shouldn't be here: a number of fbdev
>> drivers with deferred I/O contain similar code and the fbdev module should
>> provide us with a helper. (I think I even had some patches somewhere.)
> 
> Well my thinking is that it's a drm driver, so if we have issue with limit

Technically, it's not a driver, but a client.

> checks blowing up it makes more sense to check them against drm limits.

You can still do this in fb_dirty during the actual update.

> Plus a lot more people understand those than fbdev. They should all match
> anyway, or if they dont, we have a bug. The thing is, if you change this
> further to just pass the drm_framebuffer, then this 100% becomes a drm
> function, which could be used by anything in drm really.

The code is only useful to fbdev-generic and a number older of fbdev 
drivers. Nothing else uses deferred I/O and the rsp damage handling. 
Anything else operates directly on memory buffers. (i915 is a bit 
special, but doesn't use this damage handling.)  The function really 
should not be here.

I'd push back on additional DRM code with fbdev deferred I/O. Drivers' 
fbdev should either operate directly on GEM buffers; or the driver 
should employ fbdev-generic.

Best regards
Thomas

> 
> But also *shrug*.
> -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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