Re: [PATCH v2 0/4] PR_SET_NO_NEW_PRIVS, unshare, and chroot

[Andy Lutomirski]

On Mon, Jan 16, 2012 at 12:49 PM, Colin Walters <walters@xxxxxxxxxx> wrote:
> On Sun, 2012-01-15 at 16:37 -0800, Andy Lutomirski wrote:
>> To make the no_new_privs discussion more concrete, here is an updated
>> series that is actually useful.  It adds PR_SET_NO_NEW_PRIVS
>
> I think it'd be clearer to call it PR_SET_NOSUID - basically it should
> match the semantics for MS_NOSUID mounts, as if on every exec()
> thereafter the target binary was on a nosuid filesystem.

The MS_NOSUID semantics are somewhat ridiculous for selinux, and I'd
rather not make them match for no_new_privs.  AppArmor completely
ignores MS_NOSUID, so I think the rename would just cause more
confusion.

>
> You might then change this flag to only take effect on a later exec(),
> which would solve your race condition for the hypothetical PAM module.

That would just make it more complicated.  The race is already solved
in the current patch, anyway.

>
>>  with the
>> same semantics as before (plus John Johansen's AppArmor fix and with
>> improved bisectability).  It then allows some unshare flags
>
> What's the rationale behind the unshare subset?  Did you actually
> analyze any setuid binaries found on Debian/Fedora etc. and determined
> that e.g. CLONE_NEWNET was problematic for some reason?

CLONE_NEWNET seems more likely to consume significant kernel resources
than the others.  I didn't have a great reason, though.  Unsharing the
filesystem namespace is possibly dangerous because it could prevent an
unmount in the original namespace from taking effect everywhere.

>
> I actually want CLONE_NEWNET for my build tool, so I can be sure the
> arbitrary code I'm executing as part of the build at least isn't
> downloading more new code.
>
>

Fair enough.  I may add this in v3.  seccomp is an even better
solution, though :)

--Andy
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