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

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

But also *shrug*.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch