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

[Serge E. Hallyn]

Quoting Matt Brown (matt at nmatt.com):
> On 6/1/17 5:24 PM, Alan Cox wrote:
> >> There's a difference between "bugs" and "security bugs". Letting
> > 
> > Not really, it's merely a matter of severity of result. A non security
> > bug that hoses your hard disk is to anyone but security nutcases at
> > least as bad as a security hole.
> > 
> >> security bugs continue to get exploited because we want to flush out
> >> bugs seems insensitive to the people getting attacked. I'd rather
> >> protect against a class of bug than have to endless fix each bug.
> > 
> > The others are security bugs too to varying degree
> > 
> >>> I'm not against doing something to protect the container folks, but that
> >>> something as with Android is a whitelist of ioctls. And if we need to do
> >>> this with a kernel hook lets do it properly.
> >>>
> >>> Remember the namespace of the tty on creation
> >>> If the magic security flag is set then
> >>>         Apply a whitelist to *any* tty ioctl call where the ns doesn't
> >>>                 match
> >>>
> >>> and we might as well just take the Android whitelist since they've kindly
> >>> built it for us all!
> >>>
> >>> In the tty layer it ends up being something around 10 lines of code and
> >>> some other file somewhere in security/ that's just a switch or similar
> >>> with the whitelisted ioctl codes in it.
> >>>
> >>> That (or a similar SELinux ruleset) would actually fix the problem.
> >>> SELinux would be better because it can also apply the rules when doing
> >>> things like su/sudo/...  
> >>
> >> Just to play devil's advocate, wouldn't such a system continue to not
> >> address your physical-console concerns? I wouldn't want to limit the
> > 
> > It would for the cases that a whitelist and container check covers -
> > because the whitelist wouldn't allow you to do anything but boring stuff
> > on the tty. TIOCSTI is just one of a whole range of differently stupid
> > and annoying opportunities. Containers do not and should not be able to
> > set the keymap, change the video mode, use console selection, make funny
> > beepy noises, access video I/O registers and all the other stuff like
> > that. Nothing is going to break if we have a fairly conservative
> > whitelist.
> > 
> >> protection to only containers (but it's a good start), since it
> >> wouldn't protect people not using containers that still have a
> >> privileged TTY attached badly somewhere.
> > 
> > How are you going to magically fix the problem. I'm not opposed to fixing
> > the real problem but right now it appears to be a product of wishful
> > thinking not programming. What's the piece of security code that
> > magically discerns the fact you are running something untrusted at the
> > other end of your tty. SELinux can do it via labelling but I don't see
> > any generic automatic way for the kernel to magically work out when to
> > whitelist and when not to. If there is a better magic rule than
> > differing-namespace then provide the code.
> > 
> > You can't just disable TIOCSTI, it has users deal with it. You can
> > get away with disabling it for namespace crossing I think but if you do
> > that you need to disable a pile of others.
> > 
> > (If it breaks containers blocking TIOCSTI then we need to have a good
> > look at algorithms for deciding when to flush the input queue on exiting
> > a container or somesuch)
> > 
> >> If you're talking about wholistic SELinux policy, sure, I could
> >> imagine a wholistic fix. But for the tons of people without a
> >> comprehensive SELinux policy, the proposed protection continues to
> >> make sense.
> > 
> > No it doesn't. It's completely useless unless you actually bother to
> > address the other exploit opportunities.
> > 
> > Right now the proposal is a hack to do 
> > 
> > 	if (TIOCSTI && different_namespace && magic_flag)
> > 
> 
> 
> This is not what my patch does. Mine is like:
> 
> 	if (TIOCSTI && !ns_capable(tty->owner_user_ns, CAP_SYS_ADMIN) &&
> 		magic_flag)
> 
> in other words:
> 	if (TIOCSTI && (different_owner_user_ns || !CAP_SYS_ADMIN) &&
> 		magic_flag)
> 
> can you specify what you mean by different_namespace? which namespace?

I think you're focusing on the wrong thing.  Your capable check (apart
from the fact that I think I've been convinced CAP_SYS_ADMIN is wrong)
is fine.

The key point is to not only check for TIOCSTI, but instead check for
a whitelisted ioctl.

What would the whitelist look like?  Should configuing that be the way
that you enable/disable, instead of the sysctl in this patchset?  So
by default the whitelist includes all ioctls (no change), but things
like sandboxes/sudo/container-starts can clear out the whitelist?
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