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Message-Id: <B9C55CFA-74CE-4241-B9D0-F2087A4D35DB@amacapital.net>
Date: Thu, 21 Feb 2019 07:49:23 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Jann Horn <jannh@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>,
"Reshetova, Elena" <elena.reshetova@...el.com>,
"Perla, Enrico" <enrico.perla@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
"tytso@....edu" <tytso@....edu>
Subject: Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon system call
> On Feb 21, 2019, at 5:20 AM, Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Feb 21, 2019 at 7:38 AM Andy Lutomirski <luto@...nel.org> wrote:
>>> On Feb 20, 2019, at 2:20 PM, Kees Cook <keescook@...omium.org> wrote:
>>>
>>> On Wed, Feb 13, 2019 at 11:52 PM Reshetova, Elena
>>> <elena.reshetova@...el.com> wrote:
>>>> 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?
>>>
>>> I would agree, let's just do this.
>>
>> Thinking slightly more about this, it’s an incomplete protection. It
>> keeps an attacker from returning to kernel mode, but it does not
>> protect the privileged flag bits. I think that IOPL is the only thing
>> we really care about, and doing anything useful about IOPL would be
>> rather more complex, unfortunately. I suppose we could just zero it
>> and guard that with a static branch that is switched off the first
>> time anyone uses iopl(3).
>>
>> I suppose we could also add a config option to straight-up disable
>> IOPL. I sincerely hope that no one uses it any more. Even the small
>> number of semi-legit users really ought to be using ioperm() instead.
>
> /me raises hand. iopl(3) is useful for making CLI and STI work from
> userspace, I've used it for that (for testing stuff, not for anything
> that has been shipped to people). Of course, that's probably a reason
> to get rid of it, not to keep it. ^^
I was thinking that I don’t even try to make this use case work correctly ;)
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