Re: [PATCH v2] security: disable FORTIFY_SOURCE on clang

[George Burgess <gbiv@google.com>]

I took a bit to poke Clang here. Building an arbitrary file with
`CONFIG_FORTIFY_SOURCE=y`, none of the functions in this range
https://github.com/ClangBuiltLinux/linux/blob/0bee0cece/include/linux/string.h#L274-L468
have FORTIFY'ed definitions emitted by clang, i.e., the added FORTIFY checks
aren't helping. Happy to check other functions elsewhere if they exist,
but given that this entire block seems to be a functional nop...

Reviewed-by: George Burgess IV <gbiv@google.com>


On Tue, May 5, 2020 at 8:53 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Tue, May 05, 2020 at 07:54:09PM -0700, Kees Cook wrote:
> > On Tue, May 05, 2020 at 06:14:53PM -0600, Jason A. Donenfeld wrote:
> > > clang-10 has a broken optimization stage that doesn't allow the
> > > compiler to prove at compile time that certain memcpys are within
> > > bounds, and thus the outline memcpy is always called, resulting in
> > > horrific performance, and in some cases, excessive stack frame growth.
> > > Here's a simple reproducer:
> > >
> > >     typedef unsigned long size_t;
> > >     void *c(void *dest, const void *src, size_t n) __asm__("memcpy");
> > >     extern inline __attribute__((gnu_inline)) void *memcpy(void *dest, const void *src, size_t n) { return c(dest, src, n); }
> > >     void blah(char *a)
> > >     {
> > >       unsigned long long b[10], c[10];
> > >       int i;
> > >
> > >       memcpy(b, a, sizeof(b));
> > >       for (i = 0; i < 10; ++i)
> > >         c[i] = b[i] ^ b[9 - i];
> > >       for (i = 0; i < 10; ++i)
> > >         b[i] = c[i] ^ a[i];
> > >       memcpy(a, b, sizeof(b));
> > >     }
> > >
> > > Compile this with clang-9 and clang-10 and observe:
> > >
> > > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-10 -Wframe-larger-than=0 -O3 -c b.c -o c10.o
> > > b.c:5:6: warning: stack frame size of 104 bytes in function 'blah' [-Wframe-larger-than=]
> > > void blah(char *a)
> > >      ^
> > > 1 warning generated.
> > > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-9 -Wframe-larger-than=0 -O3 -c b.c -o c9.o
> > >
> > > Looking at the disassembly of c10.o and c9.o, one can see that c9.o is
> > > properly optimized in the obvious way one would expect, while c10.o has
> > > blown up and includes extern calls to memcpy.
> > >
> > > But actually, for versions of clang earlier than 10, fortify source
> > > mostly does nothing. So, between being broken and doing nothing, it
> > > probably doesn't make sense to pretend to offer this option. So, this
> > > commit just disables it entirely when compiling with clang.
> > >
> > > Cc: Arnd Bergmann <arnd@arndb.de>
> > > Cc: LKML <linux-kernel@vger.kernel.org>
> > > Cc: clang-built-linux <clang-built-linux@googlegroups.com>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: George Burgess <gbiv@google.com>
> > > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > > Link: https://bugs.llvm.org/show_bug.cgi?id=45802
> > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> >
> > Grudgingly,
> >
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> >
> > --
> > Kees Cook
> >
>
> I feel like you should finish your investigation into how broken this
> actually is before we give it the hammer like this but if it is going
> in regardless...
>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>