Date: Wed, 26 Apr 2017 13:47:32 +0100 From: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> To: Jann Horn <jannh@...gle.com> Cc: Matt Brown <matt@...tt.com>, serge@...lyn.com, jmorris@...ei.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, jslaby@...e.com, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, kernel-hardening@...ts.openwall.com, linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org Subject: Re: [PATCH v5 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN > open() what? As far as I know, for System-V PTYs, there is no path you can > open() that will give you the PTY master. Am I missing something? Sorry brain fade - no. > > >> > If I want to do the equvalent of the TIOCSTI attack then I fork a process > >> > and exit the parent. The child can now use ptrace to reprogram your shell > >> > to do whatever interesting things it likes (eg running child processes > >> > called "su" via a second pty/tty pair). Not exactly rocket science. > >> > >> Why would the child be able to ptrace the shell? AFAICS, in the most > >> relevant scenarios, the child can't ptrace the shell because the > >> shell has a different UID (in the case of e.g. su or sudo). In other > > > > If I am the attacker wanting to type something into your su when you go > > and su from my account, or where the user account is trojanned I do the > > following > > > > fork > > exit parent > > child ptraces the shell (same uid as it's not setuid) > > > > You type "su" return > > The modified shell opens a new pty/tty pair and runs su over it > > My ptrace hooks watch the pty/tty traffic until you go to the loo > > My ptrace hooks switch the console > > My ptrace hooks type lots of stuff and hack your machine while eating the > > output > > > > and you come back, do stuff and then exit > > > > And if you are in X it's even easier and I don't even need to care about > > sessions or anything. X has no mechanism to sanely fix the problem, but > > Wayland does. > > I think the "When using a program like su or sudo" in the patch description > refers to the usecase where you go from a more privileged context (e.g. a > root shell) to a less privileged one (e.g. a shell as a service-specific > account used to run a daemon), not the other way around. Which is the sudo case and why sudo uses a separate pty/tty pair as it's not just TIOCSTI that's an issue but there are a load of ioctls that do things like cause signals to the process or are just annoying - vhangup(), changing the speed etc (And for console changing the keymap - which is a nasty one) > [However, I do think that it's a nice side effect of this patch that it will > prevent a malicious program from directly injecting something like an > SSH command into my shell in a sufficiently hardened environment > (with LSM restrictions that prevent the malicious program from opening > SSH keyfiles or executing another program that can do that). Although > you could argue that in such a case, the LSM should be taking care of > blocking TIOCSTI.] I would submit that creating a new pty/tty pair is the proper answer for that case however. Making the tty calls respect namespaces is however still a no-brainer IMHO. Alan
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