Quoting m...@nmatt.com (m...@nmatt.com): > On 2017-04-20 11:19, Serge E. Hallyn wrote: > >Quoting Matt Brown (m...@nmatt.com): > >>On 04/19/2017 07:53 PM, Serge E. Hallyn wrote: > >>>Quoting Matt Brown (m...@nmatt.com): > >>>>On 04/19/2017 12:58 AM, Serge E. Hallyn wrote: > >>>>>On Tue, Apr 18, 2017 at 11:45:26PM -0400, Matt Brown wrote: > >>>>>>This patch reproduces GRKERNSEC_HARDEN_TTY functionality from the > >>>>>>grsecurity > >>>>>>project in-kernel. > >>>>>> > >>>>>>This will create the Kconfig SECURITY_TIOCSTI_RESTRICT and the > >>>>>>corresponding > >>>>>>sysctl kernel.tiocsti_restrict that, when activated, restrict all > >>>>>>TIOCSTI > >>>>>>ioctl calls from non CAP_SYS_ADMIN users. > >>>>>> > >>>>>>Possible effects on userland: > >>>>>> > >>>>>>There could be a few user programs that would be effected by this > >>>>>>change. > >>>>>>See: <https://codesearch.debian.net/search?q=ioctl%5C%28.*TIOCSTI> > >>>>>>notable programs are: agetty, csh, xemacs and tcsh > >>>>>> > >>>>>>However, I still believe that this change is worth it given that the > >>>>>>Kconfig defaults to n. This will be a feature that is turned on for the > >>>>> > >>>>>It's not worthless, but note that for instance before this was fixed > >>>>>in lxc, this patch would not have helped with escapes from privileged > >>>>>containers. > >>>>> > >>>> > >>>>I assume you are talking about this CVE: > >>>>https://bugzilla.redhat.com/show_bug.cgi?id=1411256 > >>>> > >>>>In retrospect, is there any way that an escape from a privileged > >>>>container with the this bug could have been prevented? > >>> > >>>I don't know, that's what I was probing for. Detecting that the pgrp > >>>or session - heck, the pid namespace - has changed would seem like a > >>>good indicator that it shouldn't be able to push. > >>> > >> > >>pgrp and session won't do because in the case we are discussing > >>current->signal->tty is the same as tty. > >> > >>This is the current check that is already in place: > >> | if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) > >> | return -EPERM; > > > >Yeah... > > > >>The only thing I could find to detect the tty message coming from a > >>container is as follows: > >> | task_active_pid_ns(current)->level > >> > >>This will be zero when run on the host, but 1 when run inside a > >>container. However this is very much a hack and could probably break > >>some userland stuff where there are multiple levels of namespaces. > > > >Yes. This is also however why I don't like the current patch, because > >capable() will never be true in a container, so nested containers > >break. > > > > What do you mean by "capable() will never be true in a container"? > My understanding > is that if a container is given CAP_SYS_ADMIN then > capable(CAP_SYS_ADMIN) will return > true?
No, capable(X) checks for X with respect to the initial user namespace. So for root-owned containers it will be true, but containers running in non-initial user namespaces cannot pass that check. To check for privilege with respect to another user namespace, you need to use ns_capable. But for that you need a user_ns to target. > I agree the hack I mentioned above would be a bad idea because > it would break > nested containers, but the current patch would not IMO. > > A better version of the hack could involve a config > CONFIG_TIOCSTI_MAX_NS_LEVEL where > a check would be performed to ensure that > task_active_pid_ns(current)->level is not > greater than the config value(an integer that is >= 0) . Yeah. That would break a different set of cases than the capable check, I assume. A smaller set, I think. > Again, I think we both would agree that this is not the best > solution. The clear > downside is that you could have multiple container layers where the > desired security > boundaries happened to fall at different levels. Just throwing ideas > around. Yup, appreciated.
