From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: thoughts on reallocarray, explicit_bzero?
Date: Wed, 28 Jan 2015 21:19:19 -0500 [thread overview]
Message-ID: <20150129021919.GM4574@brightrain.aerifal.cx> (raw)
In-Reply-To: <CAPLrYETk3dT8EnCbqts1u3FYO=iPP7SBop2J_wixk87isR7PPQ@mail.gmail.com>
On Wed, Jan 28, 2015 at 11:34:20PM +0100, Daniel Cegiełka wrote:
> 2015-01-28 23:01 GMT+01:00 Daniel Cegiełka <daniel.cegielka@gmail.com>:
> > 2014-05-19 18:16 GMT+02:00 Rich Felker <dalias@libc.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...
I see no way memset_s is technically "better". It's unable to find and
clear other temporary copies that have been made, and the barrier
method described above already reliably clears the pointed-to copy.
Rich
next prev parent reply other threads:[~2015-01-29 2:19 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-19 15:31 Isaac Dunham
2014-05-19 15:43 ` Rich Felker
2014-05-19 16:19 ` Daniel Cegiełka
2014-05-20 6:19 ` Rich Felker
2014-05-20 15:50 ` Daniel Cegiełka
2014-05-19 15:44 ` Daniel Cegiełka
2014-05-19 16:16 ` Rich Felker
2014-05-19 16:30 ` Daniel Cegiełka
2014-05-19 16:32 ` Szabolcs Nagy
2015-01-28 22:01 ` Daniel Cegiełka
2015-01-28 22:34 ` Daniel Cegiełka
2015-01-28 22:38 ` Nathan McSween
2015-01-28 22:54 ` Daniel Cegiełka
2015-01-28 23:02 ` Josiah Worcester
2015-01-29 2:19 ` Rich Felker [this message]
2015-01-29 4:03 ` Brent Cook
2015-01-29 4:15 ` Rich Felker
2015-01-29 9:30 ` Daniel Cegiełka
2015-01-29 10:04 ` Szabolcs Nagy
2015-01-29 10:31 ` Daniel Cegiełka
2015-01-29 10:54 ` Daniel Cegiełka
2014-05-19 16:25 ` Szabolcs Nagy
2014-05-19 16:45 ` Daniel Cegiełka
2014-05-19 16:58 ` Rich Felker
2014-05-19 16:55 ` Rich Felker
2014-05-19 18:12 ` Szabolcs Nagy
2014-05-19 22:08 ` Andy Lutomirski
2014-05-20 0:41 ` Szabolcs Nagy
2014-06-11 9:59 ` Thorsten Glaser
2014-06-11 12:59 ` Rich Felker
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150129021919.GM4574@brightrain.aerifal.cx \
--to=dalias@libc.org \
--cc=musl@lists.openwall.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).