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

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Message-ID: <f713cf25-1a53-f691-ad54-6141f2d4bccb@leemhuis.info>
Date:   Wed, 31 May 2023 09:51:51 +0200
From:   Thorsten Leemhuis <linux@...mhuis.info>
To:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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

Kees, thx for looking into this.

Joan, thx for bisecting. I should have done this earlier myself...

On 31.05.23 02:33, 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/

Did a test build and booted it in a Vm, everything seems fine. So thx
again and feel free to add:

Tested-by: Thorsten Leemhuis <linux@...mhuis.info>

Ciao, Thorsten