linux-hardening - Re: Disabling -Warray-bounds for gcc-13 too

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Message-ID: <529e56c85732b0bfcf277de9c651f6c58f47b3b4.camel@redhat.com>
Date:   Thu, 27 Apr 2023 18:46:36 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     Kees Cook <kees@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Ben Skeggs <bskeggs@...hat.com>, Karol Herbst <kherbst@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        nouveau@...ts.freedesktop.org, linux-hardening@...r.kernel.org,
        gustavo@...eddedor.com, qing.zhao@...cle.com
Subject: Re: Disabling -Warray-bounds for gcc-13 too

Hey Linus, Kees. Responses below

On Sun, 2023-04-23 at 13:23 -0700, Kees Cook wrote:
> On April 23, 2023 10:36:24 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > Kees,
> >  I made the mistake of upgrading my M2 Macbook Air to Fedora-38, and
> > in the process I got gcc-13 which is not WERROR-clean because we only
> > limited the 'array-bounds' warning to gcc-11 and gcc-12. But gcc-13
> > has all the same issues.
> > 
> > And I want to be able to do my arm64 builds with WERROR on still...
> > 
> > I guess it never made much sense to hope it was going to go away
> > without having a confirmation, so I just changed it to be gcc-11+.
> 
> Yeah, that's fine. GCC 13 released without having a fix for at least one (hopefully last) known array-bounds vs jump threading bug:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109071
> 
> > And one of them is from you.
> > 
> > In particular, commit 4076ea2419cf ("drm/nouveau/disp: Fix
> > nvif_outp_acquire_dp() argument size") cannot possibly be right, It
> > changes
> > 
> > nvif_outp_acquire_dp(struct nvif_outp *outp, u8 dpcd[16],
> > 
> > to
> > 
> > nvif_outp_acquire_dp(struct nvif_outp *outp, u8 dpcd[DP_RECEIVER_CAP_SIZE],
> > 
> > and then does
> > 
> >        memcpy(args.dp.dpcd, dpcd, sizeof(args.dp.dpcd));
> > 
> > where that 'args.dp.dpcd' is a 16-byte array, and DP_RECEIVER_CAP_SIZE is 15.
> 
> Yeah, it was an incomplete fix. I sent the other half here, but it fell through the cracks:
> https://lore.kernel.org/lkml/20230204184307.never.825-kees@kernel.org/

Thanks for bringing this to our attention, yeah this definitely just looks
like it got missed somewhere down the line. It looks like Karol responded
already so I assume the patch is in the pipeline now, but let me know if
there's anything else you need.

> 
> 
> 
> > 
> 
> > I think it's all entirely harmless from a code generation standpoint,
> > because the 15-byte field will be padded out to 16 bytes in the
> > structure that contains it, but it's most definitely buggy.
> 
> Right; between this, that GCC 13 wasn't released yet, and I had no feedback from NV folks, I didn't chase down landing that fix.
> 
> > 
> > So that warning does find real cases of wrong code. But when those
> > real cases are hidden by hundreds of lines of unfixable false
> > positives, we don't have much choice.
> 
> Yup, totally agreed. The false positives I've looked at all seem to be similar to the outstanding jump threading bug, so I'm hoping once that gets fixed we'll finally have a good signal with that warning enabled. :)
> 
> -Kees
> 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat