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Date: Tue, 16 Feb 2016 20:44:35 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: list of security features in musl
* Solar Designer <solar@...nwall.com> [2016-02-16 20:45:32 +0300]:
> On Thu, Feb 11, 2016 at 08:11:19PM +0100, Szabolcs Nagy wrote:
> > - about 'security feature lists':
> > the fedora project lists 'sha256 based passwd hash' in glibc
> > as a security feature[0], that implementation is
> > - a denial of service attack vector (computation depends on
> > key length more than the admin controlled round count).
> > - arch dependent(!), one can craft a passwd entry such that
> > only 32bit machines can log in.
>
> What do you mean here? 32-bit overflow/wraparound with very high
> rounds= specification?
>
no,
rounds setting is specified in terms of strtoul which has
saturating semantics so large values are not a problem
(and out of range values are clamped into [1000,999999999]).
but negative values are accepted by strtoul with different
meaning on 32 vs 64bit systems (wraparound).
(e.g. rounds=-4294967295 is clamped to 1000 vs 999999999).
of course arch dependent output is not a useful property
for a pbkdf so musl rejects negative rounds settings.
http://git.musl-libc.org/cgit/musl/tree/src/crypt/crypt_sha256.c#n211
Rich,
it seems musl has the wrong ROUNDS_MAX setting, do you
mind adding two more 9s there:
http://git.musl-libc.org/cgit/musl/commit/?id=aeaceb1fa89b865eb0bca739da9c450b5a054866
to follow the official spec:
https://www.akkadia.org/drepper/SHA-crypt.txt
(or reject large rounds so we don't generate non-portable hashes)
> > - unbounded alloca(!) use was fixed in 2012, long after
> > fedora added support for it (the reference implementation
> > in the spec still has the problem, among other issues[1]).
> > - uses arbitrary sized realloc for the global crypt state
> > even though 100 bytes would be enough (checks salt len
> > after reallocation).
> > - not standard conform c code: aligned attribute, alloca,
> > section attribute, undefined behaviour: (tmp - (char *) 0).
> > - meant to be used outside the libc, but secrets are cleared
> > with memset which can be optimized away.
> > (i think there are other issues in this sha256-crypt.c, but
> > the point is: implementation details matter so security check
> > lists should be taken with a grain of salt.)
> >
> > [0]: https://fedoraproject.org/wiki/Security_Features#Glibc_Enhancements
> > [1]: http://www.openwall.com/lists/musl/2012/08/19/9
>
> Another issue is that SHA-crypt leaks 8 bits via timing (total execution
> time, not just cache-timing), for no good reason at all (not a tradeoff):
>
> "18. repeast the following 16+A[0] times, where A[0] represents the first
> byte in digest A interpreted as an 8-bit unsigned value
>
> add the salt to digest DS"
>
and key length is leaked too :)
> For comparison, bcrypt is not cache-timing-safe, but that's a tradeoff.
>
> Alexander
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