Date: Wed, 26 Apr 2017 10:21:36 -0400 From: Matt Brown <matt@...tt.com> To: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>, Jann Horn <jannh@...gle.com> Cc: 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 On 04/26/2017 08:47 AM, One Thousand Gnomes wrote: >> 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) > Are any of these annoyances potential security issues? I would be happy to add patches or modify this one to include extra hardening measures. >> [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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