linux-kernel - Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory

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Message-ID: <87fsc1il73.fsf@oldenburg.str.redhat.com>
Date:   Mon, 23 Jan 2023 11:45:04 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com
Subject: Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory

* David Hildenbrand:

> On 19.01.23 22:23, Rick Edgecombe wrote:
>> The x86 Control-flow Enforcement Technology (CET) feature includes a new
>> type of memory called shadow stack. This shadow stack memory has some
>> unusual properties, which requires some core mm changes to function
>> properly.
>> Shadow stack memory is writable only in very specific, controlled
>> ways.
>> However, since it is writable, the kernel treats it as such. As a result
>> there remain many ways for userspace to trigger the kernel to write to
>> shadow stack's via get_user_pages(, FOLL_WRITE) operations. To make this a
>> little less exposed, block writable GUPs for shadow stack VMAs.
>> Still allow FOLL_FORCE to write through shadow stack protections, as
>> it
>> does for read-only protections.
>
> So an app can simply modify the shadow stack itself by writing to
> /proc/self/mem ?
>
> Is that really intended? Looks like security hole to me at first
> sight, but maybe I am missing something important.

Isn't it possible to overwrite GOT pointers using the same vector?
So I think it's merely reflecting the status quo.

Thanks,
Florian