Re: Docs for PR_SET_NO_NEW_PRIVS

[Will Drewry]

On Tue, Jun 5, 2012 at 12:33 PM, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> 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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