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Date: Wed, 28 Jan 2015 14:38:06 -0800
From: Nathan McSween <nwmcsween@...il.com>
To: musl@...ts.openwall.com
Subject: Re: thoughts on reallocarray, explicit_bzero?
How can a compiler optimize out a memory barrier?
On Jan 28, 2015 2:34 PM, "Daniel Cegiełka" <daniel.cegielka@...il.com>
wrote:
> 2015-01-28 23:01 GMT+01:00 Daniel Cegiełka <daniel.cegielka@...il.com>:
> > 2014-05-19 18:16 GMT+02:00 Rich Felker <dalias@...c.org>:
> >> On Mon, May 19, 2014 at 05:44:59PM +0200, Daniel Cegiełka wrote:
> >
> >>> diff -urN musl.orig/src/string/explicit_bzero.c
> musl/src/string/explicit_bzero.c
> >>> --- musl.orig/src/string/explicit_bzero.c Thu Jan 1 00:00:00 1970
> >>> +++ musl/src/string/explicit_bzero.c Fri May 9 09:57:45 2014
> >>> @@ -0,0 +1,8 @@
> >>> +#include <string.h>
> >>> +
> >>> +static void *(*volatile explicit_memset)(void *, int, size_t) =
> memset;
> >>> +
> >>> +void explicit_bzero(void *b, size_t len)
> >>> +{
> >>> + (*explicit_memset)(b, 0, len);
> >>> +}
> >>
> >> This is a nice trick, but IIRC I actually observed GCC optimizing out
> >> similar code before (instead of your static volatile, I used a
> >> volatile compound literal). At least the concept is right though: you
> >> want to prevent the compiler from being able to do any flow analysis
> >> at compile time, and making the function pointer volatile achieves
> >> this rather well. On the other hand, GCC will put the volatile pointer
> >> (if it even emits it) in non-constant memory, meaning it's an
> >> additional attack vector for function-pointer-overwrite attacks.
> >
> > Linux kernel has similar functions and uses a barrier() here:
> >
> >
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/lib/string.c?id=refs/tags/v3.19-rc6#n600
> >
> >
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/compiler.h?id=refs/tags/v3.19-rc6#n162
> >
> > Is such a solution is more correct (and still portable)?
>
> I'm afraid that the only appropriate solution is to use memset_s()
> from C11 and the expectation that the compiler will accept it.
> barrier() does not give any guarantee that this function will be
> secure. Only compiler decides. I'm afraid that OpenBSD goes bad path
> with explicit_bzero(). The same applies to the linux kernel and
> memzero_explicit().. very stupid name...
>
> Daniel
>
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