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Date:   Thu, 30 Aug 2018 18:23:15 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jannh@...gle.com>
Cc:     yu-cheng.yu@...el.com, Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omiun.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW



> On Aug 30, 2018, at 10:59 AM, Jann Horn <jannh@...gle.com> wrote:
> 
>> On Thu, Aug 30, 2018 at 7:58 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>> 
>>> On Thu, 2018-08-30 at 10:33 -0700, Dave Hansen wrote:
>>>> On 08/30/2018 10:26 AM, Yu-cheng Yu wrote:
>>>> 
>>>> We don't have the guard page now, but there is a shadow stack
>>>> token
>>>> there, which cannot be used as a return address.
>>> The overall concern is that we could overflow into a page that we
>>> did
>>> not intend.  Either another actual shadow stack or something that a
>>> page
>>> that the attacker constructed, like the transient scenario Jann
>>> described.
>>> 
>> 
>> A task could go beyond the bottom of its shadow stack by doing either
>> 'ret' or 'incssp'.  If it is the 'ret' case, the token prevents it.
>> If it is the 'incssp' case, a guard page cannot prevent it entirely,
>> right?
> 
> I mean the other direction, on "call".

I still think that shadow stacks should work just like mmap and that mmap should learn to add guard pages for all non-MAP_FIXED allocations.

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