Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon system call

From: Jann Horn <jannh@google.com>
To: "Reshetova, Elena" <elena.reshetova@intel.com>
Cc: "Perla, Enrico" <enrico.perla@intel.com>,
	Andy Lutomirski <luto@amacapital.net>,
	"keescook@chromium.org" <keescook@chromium.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"kernel-hardening@lists.openwall.com"
	<kernel-hardening@lists.openwall.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>, "tytso@mit.edu" <tytso@mit.edu>
Subject: Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon system call
Date: Tue, 19 Feb 2019 15:47:41 +0100	[thread overview]
Message-ID: <CAG48ez2xpgLhEQp7y9AJw0gS2yk1kBC7SDbOG_7Piqti-1eiNA@mail.gmail.com> (raw)
In-Reply-To: <2236FBA76BA1254E88B949DDB74E612BA4BC57C1@IRSMSX102.ger.corp.intel.com>

On Thu, Feb 14, 2019 at 8:52 AM Reshetova, Elena
<elena.reshetova@intel.com> wrote:
> After some thinking and discussions, let me try to summarize the options and the
> security benefits of each approach based on everyone else feedback before
> going any further with work. We all want only useful things in kernel, so merging
> smth that provides a subtle/almost non-existing benefit is clearly not a priority.
>
> So, first about the protection methods. We can do randomization in two ways:
>
> 1. stack top itself (and location of pt_regs)
> 2. relative stack positioning (offset from pt_regs).
>
> In addition to randomization we can do some fixup on exit, like Andy proposed:
>
> 3. Make sure CS always points to user on exit (for example by regs->cs |= 3),
> potentially smth else can be fixed up similarly, if needed
>
> Here are *known* (at least to me, please shout if you know more!) possible
> attacks vectors and implications on them by doing 1), 2) or 3)
>
> a) attacker's goal is to store some user-controlled data in pt_regs to reference it later
>   1) pt_regs is not predictable, but can be discovered in ptrace-style scenario or cache-probing.
>      If discovered, then attack succeeds as of now.
>   2) nothing changed for this type of attack, attack succeeds as of now
>   3) nothing changed for this type of attack, attack succeeds as of now
>
> b) attacker's goal is to return to userland from a syscall with CS pointing to kernel
>  1) pt_regs is not predictable, but can be discovered in ptrace-style scenario or cache-probing.
>      If discovered, then attack succeeds as of now.
>  2) nothing changed for this type of attack, attack succeeds as of now
>  3) CS changed explicitly on exit, so impossible to have this attack, *given* that the
>     change is done late enough and no races, sleeps, etc. are possible
>
> c) attacker's goal is to perform some kind of stack overflow into parts of adjusted stack
>   memory via some method. Here the main unknown is the "method".
> This vector of attack is the challenge for the current exploit writers: I guess if you can do
> it now with all the current protections for stack enabled, you get a nice defcon talk at least :)
> VLA used to be an easy way of doing it, hopefully they are gone from the main source now
> (out of tree drivers is a different story).
> Uninitialized locals might be other way, but there is work ongoing to close this (see Kees's
> patches on stackinit).
> Smth else we don’t yet know?
> Now back to our proposed countermeasures given that attacker has found a way to do
> a crafted overflow and overwrite:
>
>   1) pt_regs is not predictable, but can be discovered in ptrace-style scenario or cache-probing.
>      If discovered, then attack succeeds as of now.
>   2) relative stack offset is not predictable and randomized, cannot be probed very easily via
>       cache or ptrace. So, this is an additional hurdle on the attacker's way since stack is non-
>       deterministic now.
>   3) nothing changed for this type of attack, given that attacker's goal is not to overwrite CS
>       in adjusted pt_regs. If it is his goal, then it helps with that.
>
>
> Now summary:
>
> It would seem to me that:
>
> - regs->cs |= 3 on exit is a thing worth doing anyway, just because it is cheap, as Andy said, and it
> might make a positive difference in two out of three attack scenarios. Objections?
>
> - randomization of stack top is only worth doing in ptrace-blocked scenario.
> Do we have such scenarios left that people care about?

There are some, I think; for example, Chrome renderers on desktop
Linux can't use ptrace.