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Message-ID: <CAGXu5jLhwagUkATv=fT4p3LqPc=GQ8G51e-hzgzPXuAESmoW8g@mail.gmail.com> Date: Thu, 20 Apr 2017 23:01:54 -0700 From: Kees Cook <keescook@...omium.org> To: "Serge E. Hallyn" <serge@...lyn.com> Cc: Matt Brown <matt@...tt.com>, James Morris <jmorris@...ei.org>, Greg KH <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, linux-security-module <linux-security-module@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN On Thu, Apr 20, 2017 at 10:24 PM, Serge E. Hallyn <serge@...lyn.com> wrote: > On Fri, Apr 21, 2017 at 01:09:59AM -0400, Matt Brown wrote: >> On 04/20/2017 01:41 PM, Serge E. Hallyn wrote: >> >Quoting matt@...tt.com (matt@...tt.com): >> >>On 2017-04-20 11:19, Serge E. Hallyn wrote: >> >>>Quoting Matt Brown (matt@...tt.com): >> >>>>On 04/19/2017 07:53 PM, Serge E. Hallyn wrote: >> >>>>>Quoting Matt Brown (matt@...tt.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. >> > >> >> How about: ns_capable(current_user_ns(),CAP_SYS_ADMIN) ? >> >> current_user_ns() was found in include/linux/cred.h > > Any user can create a new user namespace and pass the above check. What we > want is to find the user namespace which opened the tty. Can we use file->cred->user_ns? Hm, but I see there isn't really a single file associated with tty_struct. -Kees -- Kees Cook Pixel Security
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