Re: [PATCH] drm/komeda: fix integer overflow in AFBC framebuffer size check

[Liviu Dudau <liviu.dudau@arm.com>]

On Tue, Feb 03, 2026 at 09:43:12PM +0000, Brian Starkey wrote:
> Hi Alexander,
> 
> On Tue, Feb 03, 2026 at 04:48:46PM +0000, Alexander Konyukhov wrote:
> > The AFBC framebuffer size validation calculates the minimum required
> > buffer size by adding the AFBC payload size to the framebuffer offset.
> > This addition is performed without checking for integer overflow.
> > 
> > If the addition oveflows, the size check may incorrectly succed and
> > allow userspace to provide an undersized drm_gem_object, potentially
> > leading to out-of-bounds memory access.
> > 
> > Add usage of check_add_overflow() to safely compute the minimum
> > required size and reject the framebuffer if an overflow is detected.
> > This makes the AFBC size validation more robust against malformed.
> > 
> > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > 
> > Fixes: 65ad2392dd6d ("drm/komeda: Added AFBC support for komeda driver")
> > Signed-off-by: Alexander Konyukhov <Alexander.Konyukhov@kaspersky.com>
> > ---
> >  drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.c b/drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.c
> > index 3ca461eb0a24..3cb34d03f7f8 100644
> > --- a/drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.c
> > +++ b/drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.c
> > @@ -4,6 +4,8 @@
> >   * Author: James.Qian.Wang <james.qian.wang@arm.com>
> >   *
> >   */
> > +#include <linux/overflow.h>
> > +
> >  #include <drm/drm_device.h>
> >  #include <drm/drm_fb_dma_helper.h>
> >  #include <drm/drm_gem.h>
> > @@ -93,7 +95,9 @@ komeda_fb_afbc_size_check(struct komeda_fb *kfb, struct drm_file *file,
> >  	kfb->afbc_size = kfb->offset_payload + n_blocks *
> >  			 ALIGN(bpp * AFBC_SUPERBLK_PIXELS / 8,
> >  			       AFBC_SUPERBLK_ALIGNMENT);
> > -	min_size = kfb->afbc_size + fb->offsets[0];
> 
> Can this really overflow? Is the concern a hypothetical ILP64
> situation?
> 
> min_size is u64, kfb->afbc_size is u32, and fb->offsets[0] is unsigned
> int.

Yeah, I was thinking the same thing yesterday at the end of the work day when I looked
at the patch. I don't think following the call flow you can end up with an overflow.

Best regards,
Liviu

> 
> Thanks,
> -Brian
> 
> > +	if (check_add_overflow(kfb->afbc_size, fb->offsets[0], &min_size)) {
> > +		goto check_failed;
> > +	}
> >  	if (min_size > obj->size) {
> >  		DRM_DEBUG_KMS("afbc size check failed, obj_size: 0x%zx. min_size 0x%llx.\n",
> >  			      obj->size, min_size);
> > -- 
> > 2.43.0
> >