On Tue, 2017-05-30 at 12:28 -0400, Matt Brown wrote:

On 5/30/17 8:24 AM, Alan Cox wrote:

quoted

Look there are two problems here

1. TIOCSTI has users

I don't see how this is a problem.

quoted

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

quoted

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).

quoted

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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