Date:   Wed, 31 May 2023 19:51:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Conor Dooley <conor@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        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, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] riscv/purgatory: Do not use fortified string functions

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@...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>
> 
> 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@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv



-- 
Kees Cook

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