linux-kernel - [PATCH v2] riscv/purgatory: Do not use fortified string functions

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Message-Id: <20230601160025.gonna.868-kees@kernel.org>
Date:   Thu,  1 Jun 2023 09:00:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Paul Walmsley <paul.walmsley@...ive.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thorsten Leemhuis <linux@...mhuis.info>,
        Joan Bruguera Micó <joanbrugueram@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alyssa Ross <hi@...ssa.is>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-riscv@...ts.infradead.org,
        Andrew Jones <ajones@...tanamicro.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2] riscv/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: Paul Walmsley <paul.walmsley@...ive.com>
Cc: Palmer Dabbelt <palmer@...belt.com>
Cc: Albert Ou <aou@...s.berkeley.edu>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Conor Dooley <conor.dooley@...rochip.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Alyssa Ross <hi@...ssa.is>
Cc: Heiko Stuebner <heiko.stuebner@...ll.eu>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-riscv@...ts.infradead.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v2: - only limit fortify for sha256 (conor)
v1: https://lore.kernel.org/lkml/20230531003404.never.167-kees@kernel.org
---
 arch/riscv/purgatory/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index 5730797a6b40..8c73360c42bb 100644
--- a/arch/riscv/purgatory/Makefile
+++ b/arch/riscv/purgatory/Makefile
@@ -31,7 +31,7 @@ $(obj)/strncmp.o: $(srctree)/arch/riscv/lib/strncmp.S FORCE
 $(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
 CFLAGS_string.o := -D__DISABLE_EXPORTS
 CFLAGS_ctype.o := -D__DISABLE_EXPORTS
 
-- 
2.34.1