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Date: Thu, 21 Feb 2019 14:20:02 +0100
From: Jann Horn <>
To: Andy Lutomirski <>
Cc: Kees Cook <>, "Reshetova, Elena" <>, 
	"Perla, Enrico" <>, Peter Zijlstra <>, 
	"" <>, 
	"" <>, "" <>, "" <>, 
	"" <>
Subject: Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon
 system call

On Thu, Feb 21, 2019 at 7:38 AM Andy Lutomirski <> wrote:
> > On Feb 20, 2019, at 2:20 PM, Kees Cook <> wrote:
> >
> > On Wed, Feb 13, 2019 at 11:52 PM Reshetova, Elena
> > <> 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. ^^

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