Date: Wed, 31 May 2017 10:32:43 -0500 From: "Serge E. Hallyn" <serge@...lyn.com> To: Alan Cox <gnomes@...rguk.ukuu.org.uk> Cc: Peter Dolding <oiaohm@...il.com>, "Serge E. Hallyn" <serge@...lyn.com>, Kees Cook <keescook@...omium.org>, Daniel Micay <danielmicay@...il.com>, Matt Brown <matt@...tt.com>, Greg KH <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>, James Morris <jmorris@...ei.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, linux-security-module <linux-security-module@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: Re: [PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN Quoting Alan Cox (gnomes@...rguk.ukuu.org.uk): > > Alan is right. CAP_SYS_ADMIN allows crossing the tty barrier. > > I don't need CAP_ anything to mmap your frame buffer, or use selection to > cut and paste text into the terminal. > > > Broken applications that you can wrap in a pty/tty pair as the lxc > > application does would be defeated if those applications move up to > > CAP_SYS_ADMIN. Because you have granted the high right of cross > > pty/tty containment. > > Yes Right. > > I don't know of a genuine program using push back in exploiting way > > where the pushed back input is expected to be processed after the > > program has terminated. > > So there are two real problems here > > 1. We don't know what namespace each character belongs to, so there's no > way we can construct a model where pushed symbols only appear in the > namespace they are pushed from. That would be a nice situation but it's > not at all obvious there is a sane way to implement it. > > 2. Focussing on TIOCSTI is just ignoring the bigger picture. TIOCSTI is > actually a lot less nasty in many situations than a framebuffer mmap and > spying attack where a container run from the console could sit and watch > you. TIOCSTI is in some ways the easiest to fix because setsid() will let > you mitigate it in certain cases whereas I'm fairly sure the selection > based console attack doesn't need controlling terminal rights. > > In the case you have a less privileged subshell you need a whole new tty > context, and there's no obvious way for the kernel to magic one into > existance so that for example the container can change it's own baud rate > but not the baud rate of any app outside the container. > > ioctl whitelisting will break stuff, but an SELinux/AppArmour/seccomp > whitelisting based solution is probably the only practical one you can > implement usefully, and for a lot of container users would be ok. Seccomp policy could refuse TIOCSTI to any fd, but that's not ideal. It could be customized per-application-start to only refuse TIOCSTI to the passed-in tty fd, but you'd have to prevent dup then right? Selinux can label the fd so it's in fact ideal here. But - is there something clever a seccomp policy could do to work only on specified fds and any that were dup'ed? Mind you, I of course agree that creating a new pty and passing only that in is the sane fix. Maybe in the end this thread will serve best as a loud reminder / teaching moment about this issue for those about to write such an application.
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