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Date: Fri, 23 Aug 2013 13:13:07 +0200
From: jca+dash@...vbn.org (Jérémie Courrèges-Anglas)
To: Tavis Ormandy <taviso@...gle.com>
Cc: Jilles Tjoelker <jilles@...ck.nl>, Harald van Dijk <harald@...awatt.nl>,
dash@...r.kernel.org, oss-security@...ts.openwall.com
Subject: Re: [PATCH] implement privmode support in dash
Tavis Ormandy <taviso@...gle.com> writes:
> On Thu, Aug 22, 2013 at 1:35 PM, Jilles Tjoelker <jilles@...ck.nl> wrote:
>> I think there is no reason to deviate from other shells here. Therefore,
>> please call it "privileged".
>>
>
> Agreed.
>
>>> In bash and FBSD, after starting with -p, set +p can be used to drop
>>> privileges. With your patch, dash accepts set +p, but silently ignores it.
>>
>>> How does something like the attached, to be applied on top of your
>>> patch, look?
>>
>>> [snip]
>>> + if (!on && (uid != geteuid() || gid != getegid())) {
>>> + setuid(uid);
>>> + setgid(gid);
>>> + /* PS1 might need to be changed accordingly. */
>>> + choose_ps1();
>>> + }
>>> +}
>>
>> This code tries to use setuid() and setgid() to drop all privilege,
>> which is only correct if the privilege to be dropped is UID 0, or on BSD
>> systems. It would be better to use setresuid() or setreuid(), and change
>> the GID before changing the UID.
>
> This is logic duplicated from pdksh and bash, I'm slightly reluctant
> to do things differently, unless it's not going to get committed
> otherwise.
pdksh is only maintained by OpenBSD, afaik (mksh syncs regularly).
The current code rather looks like this:
if (f == FPRIVILEGED && oldval && !newval) {
gid_t gid = getgid();
setresgid(gid, gid, gid);
setgroups(1, &gid);
setresuid(ksheuid, ksheuid, ksheuid);
} ...
> You can see some code snippets here:
> http://blog.cmpxchg8b.com/2013/08/security-debianisms.html
>
>> Apart from that, it is better to check the return value from setuid()
>> and similar functions. In particular, some versions of Linux may fail
>> setuid() for [EAGAIN], leaving the process running with the same
>> privileges.
>
> I don't think this is true anymore, but I have no strong objection to
> adding it, so long as it's noted that bash and pdksh do not do this.
>
> Tavis.
--
jca | PGP: 0x06A11494 / 61DB D9A0 00A4 67CF 2A90 8961 6191 8FBF 06A1 1494
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