[kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

[Stephen Smalley]

On Tue, 2017-05-30 at 12:28 -0400, Matt Brown wrote:
> On 5/30/17 8:24 AM, Alan Cox wrote:
> > Look there are two problems here
> > 
> > 1. TIOCSTI has users
> 
> I don't see how this is a problem.
> 
> > 
> > 2. You don't actually fix anything
> > 
> > The underlying problem is that if you give your tty handle to
> > another
> > process which you don't trust you are screwed. It's fundamental to
> > the
> > design of the Unix tty model and it's made worse in Linux by the
> > fact
> > that we use the tty descriptor to access all sorts of other console
> > state
> > (which makes a ton of sense).
> > 
> > Many years ago a few people got this wrong. All those apps got
> > fixes back
> > then. They allocate a tty/pty pair and create a new session over
> > that.
> > The potentially hostile other app only gets to screw itself.
> > 
> 
> Many years ago? We already got one in 2017, as well as a bunch last
> year.
> See: https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=tiocsti
> 
> > If it was only about TIOCSTI then your patch would still not make
> > sense
> > because you could use on of the existing LSMs to actually write
> > yourself
> > some rules about who can and can't use TIOCSTI. For that matter you
> > can
> > even use the seccomp feature today to do this without touching your
> > kernel because the ioctl number is a value so you can just block
> > ioctl
> > with argument 2 of TIOCSTI.
> > 
> 
> Seccomp requires the program in question to "opt-in" so to speak and
> set
> certain restrictions on itself. However as you state above, any
> TIOCSTI
> protection doesn't matter if the program correctly allocates a
> tty/pty pair.
> This protections seeks to protect users from programs that don't do
> things
> correctly. Rather than killing bugs, this feature attempts to kill an
> entire
> bug class that shows little sign of slowing down in the world of
> containers and
> sandboxes.

Just FYI, you can also restrict TIOCSTI (or any other ioctl command)
via SELinux ioctl whitelisting, and Android is using that feature to
restrict TIOCSTI usage in Android O (at least based on the developer
previews to date, also in AOSP master).

> 
> > So please explain why we need an obscure kernel config option that
> > normal
> > users will not understand which protects against nothing and can be
> > done already ?
> > 
> > Alan
> > 
> 
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