linux-kernel - Re: [kernel-hardening] Re: vDSO maximum stack usage, stack probes, and -fstack-check

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Message-ID: <CALCETrVd-f-N=O6QmtdzS7oSVkXpsJhxH+=9+0fvfFmy5Cz3WQ@mail.gmail.com>
Date:   Fri, 10 Nov 2017 14:04:13 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Hector Martin 'marcan'" <marcan@...can.st>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>, X86 ML <x86@...nel.org>
Subject: Re: [kernel-hardening] Re: vDSO maximum stack usage, stack probes,
 and -fstack-check

> On Nov 10, 2017, at 8:36 AM, Hector Martin 'marcan' <marcan@...can.st> wrote:
>
>> On 2017-11-11 01:02, Hector Martin 'marcan' wrote:
>> Not entirely sure what's going on here.
>
> Actually, if you think about it, it doesn't matter that it skips the
> first page, since it's probing one page more. That just means the caller
> will have probed the previous page. So ultimately you're just probing
> ahead of where you need to, but that should be OK.
>

The whole point is to touch the stack pages in order.  Also, I see no
guarantee that the function would touch the intermediate page before
clobbering the probed page.  You're seeing exactly that behavior, in
fact.

> --
> Hector Martin "marcan" (marcan@...can.st)
> Public Key: https://mrcn.st/pub