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Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAG48ez3ixWROuQc6WZze6qPL6q0e_gCnMU4XF11JUWziePsBJg@mail.gmail.com>
Date:   Thu, 30 Aug 2018 18:23:15 -0700
Cc:     yu-cheng.yu@intel.com, Dave Hansen <dave.hansen@linux.intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
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References: <20180830143904.3168-1-yu-cheng.yu@intel.com> <20180830143904.3168-13-yu-cheng.yu@intel.com> <CAG48ez0Rca0XsdXJZ07c+iGPyep0Gpxw+sxQuACP5gyPaBgDKA@mail.gmail.com> <079a55f2-4654-4adf-a6ef-6e480b594a2f@linux.intel.com> <CAG48ez2gHOD9hH4+0wek5vUOv9upj79XWoug2SXjdwfXWoQqxw@mail.gmail.com> <ce051b5b-feef-376f-e085-11f65a5f2215@linux.intel.com> <1535649960.26689.15.camel@intel.com> <33d45a12-513c-eba2-a2de-3d6b630e928e@linux.intel.com> <1535651666.27823.6.camel@intel.com> <CAG48ez3ixWROuQc6WZze6qPL6q0e_gCnMU4XF11JUWziePsBJg@mail.gmail.com>
To:     Jann Horn <jannh@google.com>
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> On Aug 30, 2018, at 10:59 AM, Jann Horn <jannh@google.com> wrote:
>=20
>> On Thu, Aug 30, 2018 at 7:58 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote=
:
>>=20
>>> On Thu, 2018-08-30 at 10:33 -0700, Dave Hansen wrote:
>>>> On 08/30/2018 10:26 AM, Yu-cheng Yu wrote:
>>>>=20
>>>> 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.
>>>=20
>>=20
>> 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?
>=20
> I mean the other direction, on "call".

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