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

From: Kees Cook <keescook@chromium.org>
To: Conor Dooley <conor@kernel.org>
Cc: "Paul Walmsley" <paul.walmsley@sifive.com>,
	"Thorsten Leemhuis" <linux@leemhuis.info>,
	"Joan Bruguera Micó" <joanbrugueram@gmail.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Alyssa Ross" <hi@alyssa.is>,
	"Heiko Stuebner" <heiko.stuebner@vrull.eu>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH] riscv/purgatory: Do not use fortified string functions
Date: Wed, 31 May 2023 19:51:10 -0700	[thread overview]
Message-ID: <202305311950.EFFD6521@keescook> (raw)
In-Reply-To: <20230531-abruptly-settling-f9852f408dcd@spud>

On Wed, May 31, 2023 at 11:37:20PM +0100, Conor Dooley wrote:
> Hey Kees,
> 
> On Tue, May 30, 2023 at 05:34:04PM -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 <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>
> 
> Perhaps this is indicative of other issues in RISC-V land, but
> allmodconfig complains about this patch:
> 
> ../lib/string.c:17: warning: "__NO_FORTIFY" redefined
> ../lib/string.c:17:9: warning: preprocessor token __NO_FORTIFY redefined

Argh, thanks. My compile test clearly failed. I'll fix this up.

-Kees

> 
> > ---
> >  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
> > 
> > 
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv

-- 
Kees Cook