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Date: Thu, 9 Aug 2012 01:48:04 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: crypt* files in crypt directory

On Thu, Aug 09, 2012 at 08:04:32AM +0400, Solar Designer wrote:
> On Wed, Aug 08, 2012 at 11:25:27PM -0400, Rich Felker wrote:
> > On Thu, Aug 09, 2012 at 05:51:04AM +0400, Solar Designer wrote:
> > > On Wed, Aug 08, 2012 at 09:06:23AM -0400, Rich Felker wrote:
> > > > Actually this brings up a HUGE DoS vuln in blowfish crypt: with tcb
> > > > passwords, a malicious user can put a password with count=31 (it's
> > > > logarithmic, so this means 2^31) in their tcb shadow file.
> > > 
> > > Yes, but only after having compromised group shadow.  If a user does
> > > compromise group shadow, I'd appreciate learning of that - even if via
> > > being DoS'ed. ;-)
> 
> I think I need to clarify that the above is my own preference and some
> sysadmin's preference might be different.  This is a reason why we deal
> with whatever DoS attacks may be easily dealt with anyway - those with
> non-regular files.
> 
> For DoS via high iteration count, I see no good solution other than to
> accept this as a possibility for when group shadow is compromised.

Well it's also a possibility if you're using crypt to validate
passwords where both the hash and password are provided by a third
party. I think that's a major problem. I generally frown upon
interfaces where the run time is non-obviously superlinear in the
input size.

I don't see any down-size to limiting the iteration count if the limit
is reasonable. For instance if the limit were such that higher counts
would take more than 1 second on a theoretical 50 GHz variant of a
modern cpu (which is faster than a single core will EVER be able to
get), there's no way they would be practical to use, and there's no
sense in supporting them except to satisfy a fetish for "no arbitrary
limits" even when it conflicts with security and robustness. This
would at least ensure the function can't get stuck running for
hours/days/weeks at a time.

The hard part is putting the limit at some point a good bit lower.

> > OK, so your intent is to require sgid-shadow utilities to update
> > passwords?
> 
> Yes.
> 
> /usr/bin/passwd and (if enabled) /usr/bin/chage on Owl are SGID shadow.

If reading your own password hash also requires sgid-shadow, then
screen is sgid-shadow. Which means any user can easily get full shadow
group perms (since screen is full of vulns if it's running suid/sgid)
and thus you might as well not have had the group protection to begin
with. Same applies to things like xlock.

> That's our preference (for Owl) too.  I think you misunderstood our use
> of group shadow.  It does not "cause other users' accounts to be
> compromised" if it is compromised.

Indeed, I did misunderstand. Your way is not as bad as it could be,
but I'm still not very comfortable with the security implications of
it..

> > There's no reason applications should not be able to assume they can
> > safely call crypt where both the hash/salt/setting and key were
> > provided by an untrusted party.
> 
> There is such a reason for almost two decades now - since BSDi's
> introduction of iteration counts in "setting" strings in 1993 or so.
> That's the reality by now, and I think we should not be trying to change
> it in musl.

Thanks for bringing this up. I think the same limiting logic is
required there. And if there's going to be a way to configurably
override the limit, it should be mostly/entirely shareable.

> As to untrusted keys (passwords/phrases), I agree with you.  Those kinds
> of issues should be avoided.  glibc's uses of alloca() in SHA-crypt were
> recently patched for that reason.

Good analogy. And I think this one likewise needs to be addressed and
fixed.

Rich

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