Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sun, 12 May 2019 10:02:45 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        David Laight <David.Laight@aculab.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Eric Biggers <ebiggers3@gmail.com>,
        "ebiggers@google.com" <ebiggers@google.com>,
        "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        "jannh@google.com" <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Message-ID: <20190512080245.GA7827@gmail.com>
References: <2236FBA76BA1254E88B949DDB74E612BA4C6F523@IRSMSX102.ger.corp.intel.com>
 <e4fbad8c51284a0583b98c52de4a207d@AcuMS.aculab.com>
 <2236FBA76BA1254E88B949DDB74E612BA4C760A7@IRSMSX102.ger.corp.intel.com>
 <20190508113239.GA33324@gmail.com>
 <2236FBA76BA1254E88B949DDB74E612BA4C762F7@IRSMSX102.ger.corp.intel.com>
 <20190509055915.GA58462@gmail.com>
 <2236FBA76BA1254E88B949DDB74E612BA4C7741F@IRSMSX102.ger.corp.intel.com>
 <20190509084352.GA96236@gmail.com>
 <CALCETrV1067Es=KEjkz=CtdoT79a2EJg4dJDae6oGDiTaubL1A@mail.gmail.com>
 <201905111703.5998DF5F@keescook>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <201905111703.5998DF5F@keescook>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


* Kees Cook <keescook@chromium.org> wrote:

> On Sat, May 11, 2019 at 03:45:19PM -0700, Andy Lutomirski wrote:
> > ISTM maybe a better first step would be to make get_random_bytes() be
> > much faster? :)
> 
> I'm not opposed to that, but I want to make sure we don't break it for
> "real" crypto uses...

I'm quite sure Andy implied that.

> I still think just using something very simply like rdtsc would be good 
> enough.
>
> This isn't meant to be a perfect defense: it's meant to disrupt the 
> ability to trivially predict (usually another thread's) stack offset. 

But aren't most local kernel exploit attacks against the current task? 
Are there any statistics about this?

> And any sufficiently well-positioned local attacker can defeat this no 
> matter what the entropy source, given how small the number of bits 
> actually ends up being, assuming they can just keep launching whatever 
> they're trying to attack. (They can just hold still and try the same 
> offset until the randomness aligns: but that comes back to us also 
> needing a brute-force exec deterance, which is a separate subject...)
> 
> The entropy source bikeshedding doesn't seem helpful given how few bits 
> we're dealing with.

The low number of bits is still useful in terms of increasing the 
probability of crashing the system if the attacker cannot guess the stack 
offset.

With 5 bits there's a ~96.9% chance of crashing the system in an attempt, 
the exploit cannot be used for a range of attacks, including spear 
attacks and fast-spreading worms, right? A crashed and inaccessible 
system also increases the odds of leaving around unfinished attack code 
and leaking a zero-day attack.

Thanks,

	Ingo