linux-hardening - Re: [PATCH] x86/purgatory: Do not use fortified string functions

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Date:   Thu, 1 Jun 2023 09:50:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Thorsten Leemhuis <linux@...mhuis.info>,
        Joan Bruguera Micó <joanbrugueram@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Alyssa Ross <hi@...ssa.is>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/purgatory: Do not use fortified string functions

On Thu, Jun 01, 2023 at 09:45:57AM -0700, Dave Hansen wrote:
> On 5/30/23 17:33, Kees Cook wrote:
> > With the addition of -fstrict-flex-arrays=3, struct sha256_state's
> > trailing array is no longer ignored by CONFIG_FORTIFY_SOURCE:
> > 
> > struct sha256_state {
> >         u32 state[SHA256_DIGEST_SIZE / 4];
> >         u64 count;
> >         u8 buf[SHA256_BLOCK_SIZE];
> > };
> > 
> > This means that the memcpy() calls with "buf" as a destination in
> > sha256.c's code will attempt to perform run-time bounds checking, which
> > could lead to calling missing functions, specifically a potential
> > WARN_ONCE, which isn't callable from purgatory.
> > 
> > Reported-by: Thorsten Leemhuis <linux@...mhuis.info>
> > Closes: https://lore.kernel.org/lkml/175578ec-9dec-7a9c-8d3a-43f24ff86b92@leemhuis.info/
> > Bisected-by: "Joan Bruguera Micó" <joanbrugueram@...il.com>
> > Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> 
> Hi Folks,
> 
> The -fstrict-flex-arrays=3 commit isn't upstream yet, right?  That makes

Correct.

> it a _bit_ wonky for us to carry this on the x86 side since among other
> things, the Fixes commit doesn't exist.  I'd be fine if this goes up
> along with the -fstrict-flex-arrays=3 code, so:
> 
> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>

That would be perfect; thank you! I've added it to my tree.

-- 
Kees Cook