linux-kernel - RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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Date:   Thu, 1 Aug 2019 06:35:47 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     'Kees Cook' <keescook@...omium.org>,
        'Ingo Molnar' <mingo@...nel.org>,
        'Andy Lutomirski' <luto@...nel.org>
CC:     'Theodore Ts'o' <tytso@....edu>,
        'David Laight' <David.Laight@...lab.com>,
        'Eric Biggers' <ebiggers3@...il.com>,
        "'ebiggers@...gle.com'" <ebiggers@...gle.com>,
        "'herbert@...dor.apana.org.au'" <herbert@...dor.apana.org.au>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Daniel Borkmann' <daniel@...earbox.net>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'jpoimboe@...hat.com'" <jpoimboe@...hat.com>,
        "'jannh@...gle.com'" <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "'mingo@...hat.com'" <mingo@...hat.com>,
        "'bp@...en8.de'" <bp@...en8.de>,
        "'tglx@...utronix.de'" <tglx@...utronix.de>,
        "'gregkh@...uxfoundation.org'" <gregkh@...uxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        'Peter Zijlstra' <a.p.zijlstra@...llo.nl>
Subject: RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

>> The in-stack randomization is really a very small change both code wise and
>> logic wise.
>> It does not affect real workloads and does not require enablement of other
>> features (such as GCC plugins).
>> So, I think we should really reconsider its inclusion.

>I'd agree: the code is tiny and while the benefit can't point to a
>specific issue, it does point to the general weakness of the stack
>offset being predictable which has been a core observation for many
>stack-based attacks.

>If we're going to save state between syscalls (like the 4096 random
>bytes pool), how about instead we just use a single per-CPU long mixed
>with rdtsc saved at syscall exit. That should be a reasonable balance
>between all the considerations and make it trivial for the feature to
>be a boot flag without the extra page of storage, etc.

Sounds like a viable compromise for me. 
Ingo, Andy? 

Best Regards,
Elena.