linux-kernel - Re: [PATCH v7 02/39] prctl: arch-agnostic prctl for shadow stack

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Date:   Wed, 13 Dec 2023 11:43:49 -0800
From:   Deepak Gupta <debug@...osinc.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Florian Weimer <fweimer@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v7 02/39] prctl: arch-agnostic prctl for shadow stack

On Wed, Dec 13, 2023 at 5:37 AM Mark Brown <broonie@...nel.org> wrote:
>
> On Tue, Dec 12, 2023 at 04:50:38PM -0800, Deepak Gupta wrote:
>
> > A theoretical scenario (no current workloads should've this case
> > because no shadow stack)
>
> > - User mode did _ENABLE on the main thread. Shadow stack was allocated
> > for the current
> >   thread.
> > - User mode created a bunch worker threads to run untrusted contained
> > code. They shadow
> >   stack too.
> > - main thread had to do dlopen and now need to disable shadow stack on
> > itself due to
> >   incompatibility of incoming object in address space.
> > - main thread controls worker threads and knows they're contained and
> > should still be running
> >   with a shadow stack. Although once in a while the main thread needs
> > to perform writes to a shadow
> >   stack of worker threads for some fixup (in the same addr space).
> > main thread doesn't want to delegate
> >   this responsibility of ss writes to worker threads because they're untrusted.
>
> > How will it do that (currently _ENABLE is married to _WRITE and _PUSH) ?
>
> That's feeling moderately firmly into "don't do that" territory to be
> honest, the problems of trying to modify the stack of another running
> thread while it's active just don't seem worth it - if you're
> coordinating enough to do the modifications it's probably possible to
> just ask the thread who's stack is being modified to do the modification
> itself and having an unprotected thread writing into shadow stack memory
> doesn't feel great.
>

Yeah no leanings on my side. Just wanted to articulate this scenario.
Since this is new ground,
we can define what's appropriate. Let's keep it this way where a
thread can write to shadow
stack mappings only when it itself has shadow stack enabled.