linux-kernel - [PATCH] s390/purgatory: Do not use fortified string functions

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Message-Id: <20230531003414.never.050-kees@kernel.org>
Date:   Tue, 30 May 2023 17:34:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thorsten Leemhuis <linux@...mhuis.info>,
        Joan Bruguera Micó <joanbrugueram@...il.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH] s390/purgatory: Do not use fortified string functions

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: Heiko Carstens <hca@...ux.ibm.com>
Cc: Vasily Gorbik <gor@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc: Sven Schnelle <svens@...ux.ibm.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Linux Kernel Functional Testing <lkft@...aro.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-s390@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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