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

[Alexander Konyukhov <Alexander.Konyukhov@kaspersky.com>]

Thank you for the replies.

According to ISO 9899 6.3.1 both operands are first converted to a common type (u32), there are no defined limits of kfb->afbc_size and fb->offsets[0] , so min_size can have an overflowed u32 value.

-----Original Message-----
From: Liviu Dudau <liviu.dudau@arm.com> 
Sent: Wednesday, February 4, 2026 4:25 PM
To: Brian Starkey <brian.starkey@arm.com>
Cc: Alexander Konyukhov <Alexander.Konyukhov@kaspersky.com>; Maarten Lankhorst <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>; Thomas Zimmermann <tzimmermann@suse.de>; David Airlie <airlied@gmail.com>; Simona Vetter <simona@ffwll.ch>; dri-devel@lists.freedesktop.org; linux-kernel@vger.kernel.org; lvc-project@linuxtesting.org; nd@arm.com
Subject: Re: [PATCH] drm/komeda: fix integer overflow in AFBC framebuffer size check

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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
> >