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Re: Docs for PR_SET_NO_NEW_PRIVS

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Hi,

As-is, this could probably live in
Documentation/security/no-new-privs.txt (maybe with some examples
added).

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.)

-Kees

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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