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From:   Deepak Gupta <debug@rivosinc.com>
Date:   Wed, 13 Dec 2023 11:43:49 -0800
Message-ID: <CAKC1njRHs0R=VKfn4jBap9__oR0rBHmNy7_tqHR8=xEHdUE4+A@mail.gmail.com>
Subject: Re: [PATCH v7 02/39] prctl: arch-agnostic prctl for shadow stack
To:     Mark Brown <broonie@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, Oleg Nesterov <oleg@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Shuah Khan <shuah@kernel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>,
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On Wed, Dec 13, 2023 at 5:37=E2=80=AFAM Mark Brown <broonie@kernel.org> wro=
te:
>
> 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 un=
trusted.
>
> > 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.