linux-kernel - Re: [PATCH v3] crypto: more robust crypto

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 26 Nov 2013 20:27:33 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Cesar Eduardo Barros <cesarb@...arb.eti.br>
CC:	linux-crypto@...r.kernel.org,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	James Yonan <james@...nvpn.net>,
	Florian Weimer <fw@...eb.enyo.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] crypto: more robust crypto_memneq

On 11/26/2013 01:00 AM, Cesar Eduardo Barros wrote:
> Disabling compiler optimizations can be fragile, since a new
> optimization could be added to -O0 or -Os that breaks the assumptions
> the code is making.
>
> Instead of disabling compiler optimizations, use a dummy inline assembly
> (based on RELOC_HIDE) to block the problematic kinds of optimization,
> while still allowing other optimizations to be applied to the code.
>
> The dummy inline assembly is added after every OR, and has the
> accumulator variable as its input and output. The compiler is forced to
> assume that the dummy inline assembly could both depend on the
> accumulator variable and change the accumulator variable, so it is
> forced to compute the value correctly before the inline assembly, and
> cannot assume anything about its value after the inline assembly.
>
> This change should be enough to make crypto_memneq work correctly (with
> data-independent timing) even if it is inlined at its call sites. That
> can be done later in a followup patch.
>
> Compile-tested on x86_64.

Actually with yet another version, I hoped that the "compile-tested"-only
statement would eventually disappear, ohh well. ;)

> Signed-off-by: Cesar Eduardo Barros <cesarb@...arb.eti.br>

Resolving the OPTIMIZER_HIDE_VAR() macro for others than GCC jnto a
barrier() seems a bit suboptimal, but assuming 99% of people will use
GCC anyway, then for the minority of the remaining, they will worst case
have a clever compiler and eventually mimic memcmp() in some situations,
or have a not-so-clever compiler and execute the full code as is.

Anyway, I think still better than the rather ugly Makefile workaround
imho, so I'm generally fine with this.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/