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Message-ID: <CAG48ez31W30Qqmf5ZZ1_eVQC0uobBrva3_zgRdzejztFLzKoPg@mail.gmail.com> Date: Tue, 25 Apr 2017 23:44:50 +0200 From: Jann Horn <jannh@...gle.com> To: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> 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 On Tue, Apr 25, 2017 at 11:21 PM, One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> wrote: >> Really? By "pty", are you referring to the master? If so, as far as I know, >> to go from the slave to the master, you need one of: >> >> - ptrace access to a process that already has an FD to the master, via >> ptrace() or so (/proc/$pid/fd/$fd won't work) >> - for a BSD PTY (which AFAIK isn't used much anymore), access to >> /dev/ptyXX > > fstat() and then open *assuming* I have permissions. 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? >> > 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. [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.]
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