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

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Message-Id: <2361d4e2-c8c9-4dc4-b925-ab6543ba3404@app.fastmail.com>
Date:   Mon, 06 Mar 2023 10:57:06 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "Borislav Petkov" <bp@...en8.de>
Cc:     "David Hildenbrand" <david@...hat.com>,
        "Balbir Singh" <bsingharora@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Eugene Syromiatnikov" <esyr@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Nadav Amit" <nadav.amit@...il.com>,
        "Jann Horn" <jannh@...gle.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
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        "Thomas Gleixner" <tglx@...utronix.de>,
        "Schimpe, Christina" <christina.schimpe@...el.com>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
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        "Ingo Molnar" <mingo@...hat.com>,
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        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        "Cyrill Gorcunov" <gorcunov@...il.com>
Subject: Re: [PATCH v7 24/41] mm: Don't allow write GUPs to shadow stack memory

On Mon, Mar 6, 2023, at 10:33 AM, Edgecombe, Rick P wrote:
> On Mon, 2023-03-06 at 10:15 -0800, Andy Lutomirski wrote:
>> 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?
>
> Yes, GDB shadow stack support uses it via both ptrace poke and
> /proc/pid/mem apparently. So some ability to write through is needed
> for debuggers. But not CRIU actually. It uses WRSS.
>
> There was also some discussion[0] previously about how apps might
> prefer to block /proc/self/mem for general security reasons. Blocking
> shadow stack writes while you allow text writes is probably not that
> impactful security-wise. So I thought it would be better to leave the
> logic simpler. Then when /proc/self/mem could be locked down per the
> discussion, shadow stack can be locked down the same way.

Ah, I am guilty of reading your changelog but not the code.

You said:

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.

I read that as *denying* FOLL_FORCE.  Maybe clarify the changelog?

>
> [0] 
> https://lore.kernel.org/lkml/E857CF98-EEB2-4F83-8305-0A52B463A661@kernel.org/