linux-kernel - Re: [PATCH v7 24/41] mm: Don't allow write GUPs to shadow stack memory

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Message-ID: <CALCETrViDpTbuBk+9wQa-PNzKZerwk=4MmKMXw2v3Ysxuv2k3A@mail.gmail.com>
Date:   Mon, 6 Mar 2023 10:15:13 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
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>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...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,
        david@...hat.com, debug@...osinc.com
Subject: Re: [PATCH v7 24/41] mm: Don't allow write GUPs to shadow stack memory

On Mon, Mar 6, 2023 at 5:10 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Feb 27, 2023 at 02:29:40PM -0800, 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

Is there an alternate mechanism, or do we still want to allow
FOLL_FORCE so that debuggers can write it?

--Andy