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

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Message-ID: <3c67fdd6-bce9-f695-a6c5-0fe9209bd3e4@intel.com>
Date:   Thu, 1 Jun 2023 09:45:57 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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 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
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>

We could also pick it up from the x86 side, but I think that would need
a _bit_ of a different commit message to allude to it being to prepare
for the _future_ setting of -fstrict-flex-arrays=3 and having no
practical benefits now.

Let me know if you don't want to send this up with the
-fstrict-flex-arrays=3 set.