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Date:   Wed, 31 May 2023 07:33:42 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Heiko Carstens <hca@linux.ibm.com>,
        Thorsten Leemhuis <linux@leemhuis.info>,
        Joan Bruguera =?iso-8859-1?Q?Mic=F3?= <joanbrugueram@gmail.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Kernel Functional Testing <lkft@linaro.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] s390/purgatory: Do not use fortified string functions
Message-ID: <20230531143342.GA2250333@dev-arch.thelio-3990X>
References: <20230531003414.never.050-kees@kernel.org>
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On Tue, May 30, 2023 at 05:34:15PM -0700, 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@leemhuis.info>
> Closes: https://lore.kernel.org/lkml/175578ec-9dec-7a9c-8d3a-43f24ff86b92@leemhuis.info/
> Bisected-by: "Joan Bruguera Mic?" <joanbrugueram@gmail.com>
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Linux Kernel Functional Testing <lkft@linaro.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-s390@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reading https://lore.kernel.org/202305301658.BF6ECF65C@keescook/ was
some good additional context.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/s390/purgatory/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/s390/purgatory/Makefile b/arch/s390/purgatory/Makefile
> index 32573b4f9bd2..cf14740abd1c 100644
> --- a/arch/s390/purgatory/Makefile
> +++ b/arch/s390/purgatory/Makefile
> @@ -10,7 +10,7 @@ PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
>  $(obj)/sha256.o: $(srctree)/lib/crypto/sha256.c FORCE
>  	$(call if_changed_rule,cc_o_c)
>  
> -CFLAGS_sha256.o := -D__DISABLE_EXPORTS
> +CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
>  
>  $(obj)/mem.o: $(srctree)/arch/s390/lib/mem.S FORCE
>  	$(call if_changed_rule,as_o_S)
> -- 
> 2.34.1
>