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: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Linux Kernel Functional Testing <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: "Gustavo A. R. Silva" <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
---
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
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 <[email protected]>
> Closes: https://lore.kernel.org/lkml/[email protected]/
> Bisected-by: "Joan Bruguera Mic?" <[email protected]>
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Cc: Heiko Carstens <[email protected]>
> Cc: Vasily Gorbik <[email protected]>
> Cc: Alexander Gordeev <[email protected]>
> Cc: Christian Borntraeger <[email protected]>
> Cc: Sven Schnelle <[email protected]>
> Cc: Masahiro Yamada <[email protected]>
> Cc: Linux Kernel Functional Testing <[email protected]>
> Cc: Nathan Chancellor <[email protected]>
> Cc: "Gustavo A. R. Silva" <[email protected]>
> Cc: [email protected]
> Signed-off-by: Kees Cook <[email protected]>
Reading https://lore.kernel.org/202305301658.BF6ECF65C@keescook/ was
some good additional context.
Reviewed-by: Nathan Chancellor <[email protected]>
> ---
> 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
>
On Tue, 30 May 2023 17:34:15 -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];
> };
>
> [...]
Applied to for-next/hardening, thanks!
[1/1] s390/purgatory: Do not use fortified string functions
https://git.kernel.org/kees/c/d11f44513655
--
Kees Cook