[PATCH] riscv/purgatory: Do not use fortified string functions

[Kees Cook <keescook@chromium.org>]

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 Micó" <joanbrugueram@gmail.com>
Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Conor Dooley <conor.dooley@microchip.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Alyssa Ross <hi@alyssa.is>
Cc: Heiko Stuebner <heiko.stuebner@vrull.eu>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/riscv/purgatory/Makefile | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index 5730797a6b40..11f4c275f141 100644
--- a/arch/riscv/purgatory/Makefile
+++ b/arch/riscv/purgatory/Makefile
@@ -31,9 +31,9 @@ $(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_string.o := -D__DISABLE_EXPORTS
-CFLAGS_ctype.o := -D__DISABLE_EXPORTS
+CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
+CFLAGS_string.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
+CFLAGS_ctype.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
 
 # When linking purgatory.ro with -r unresolved symbols are not checked,
 # also link a purgatory.chk binary without -r to check for unresolved symbols.
-- 
2.34.1