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 <[email protected]>
Closes: https://lore.kernel.org/lkml/[email protected]/
Bisected-by: "Joan Bruguera Micó" <[email protected]>
Fixes: df8fc4e ("kbuild: Enable -fstrict-flex-arrays=3")
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Conor Dooley <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Alyssa Ross <[email protected]>
Cc: Heiko Stuebner <[email protected]>
Cc: "Gustavo A. R. Silva" <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Conor Dooley <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Acked-by: Palmer Dabbelt <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

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