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Date: Tue, 5 Jun 2012 13:13:05 -0500
From: Will Drewry <wad@...omium.org>
To: Kees Cook <keescook@...omium.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>,
linux-security-module@...r.kernel.org, linux-arch@...r.kernel.org,
linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com, hpa@...or.com,
mingo@...hat.com, peterz@...radead.org, tglx@...utronix.de, eparis@...hat.com,
serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com,
indan@....nu, pmoore@...hat.com, corbet@....net, eric.dumazet@...il.com,
markus@...omium.org, coreyb@...ux.vnet.ibm.com, jmorris@...ei.org,
linux-man@...r.kernel.org, Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: Docs for PR_SET_NO_NEW_PRIVS
On Tue, Jun 5, 2012 at 12:33 PM, Kees Cook <keescook@...omium.org> wrote:
> Hi,
>
> As-is, this could probably live in
> Documentation/security/no-new-privs.txt (maybe with some examples
> added).
Or Documentation/prctl/no-new-privs.txt
Just a decision between what it does and how you get to it, but I'd
think either would make sense!
> As for a manpage section, I think Michael Kerrisk would happily add a
> section for PR_[SG]ET_NO_NEW_PRIVS to prctl if this could be
> summarized into a paragraph or two.
>
> (And this reminds me I should send an update for the seccomp section
> in the prctl manpage too.)
Faster on the draw than me - thanks!
>
> On Tue, Jun 5, 2012 at 10:04 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> Hi all-
>>
>> As promised (although belatedly), I wrote up some proposed documentation
>> for the no_new_privs feature. What should I do with it? I don't speak
>> groff/troff/whatever man pages are written in.
>>
>> I would be happy to license this text appropriately for whatever tree
>> it might end up in. In the mean time, it's GPLv2+.
>>
>> --- cut here ---
>>
>> The execve system call can grant a newly-started program privileges
>> that its parent did not have. The most obvious examples are
>> setuid/setgid programs and file capabilities. To prevent the parent
>> program from gaining these privileges as well, the kernel and user
>> code must be careful to prevent the parent from doing anything that
>> could subvert the child. For example:
>>
>> - The dynamic loader handles LD_* environment variables differently
>> if a program is setuid.
>> - chroot is disallowed to unprivileged processes, since it would
>> allow /etc/passwd to be replaced from the point of view of a process
>> that inherited chroot.
>> - The exec code has special handling for ptrace.
>>
>> These are all ad-hoc fixes. The no_new_privs bit (since Linux 3.5) is
>> a new, generic mechanism to make it safe for a process to modify its
>> execution environment in a manner that persists across execve. Any
>> task can set no_new_privs. Once the bit is set, it is inherited
>> across fork, clone, and execve and cannot be unset. With no_new_privs
>> set, execve promises not to grant the privilege to do anything that
>> could not have been done without the execve call. For example, the
>> setuid and setgid bits will no longer change the uid or gid; file
>> capabilities will not add to the permitted set, and LSMs will not
>> relax constraints after execve.
>>
>> Note that no_new_privs does not prevent privilege changes that do not
>> involve execve. An appropriately privileged task can still call
>> setuid(2) and receive SCM_RIGHTS datagrams.
>>
>> There are two main use cases for no_new_privs so far:
>>
>> - Filters installed for the seccomp mode 2 sandbox persist across
>> execve and can change the behavior of newly-executed programs.
>> Unprivileged users are therefore only allowed to install such filters
>> if no_new_privs is set.
>>
>> - By itself, no_new_privs can be used to reduce the attack surface
>> available to an unprivileged user. If everything running with a given
>> uid has no_new_privs set, then that uid will be unable to escalate its
>> privileges by directly attacking setuid, setgid, and fcap-using
>> binaries; it will need to compromise something without the
>> no_new_privs bit set first.
>>
>> In the future, other potentially dangerous kernel features could
>> become available to unprivileged tasks if no_new_privs is set. In
>> principle, several options to unshare(2) and clone(2) would be safe
>> when no_new_privs is set, and no_new_privs + chroot is considerable
>> less dangerous than chroot by itself.
>>
>> --- cut here ---
>>
>> --Andy
>
>
>
> --
> Kees Cook
> Chrome OS Security
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