linux-kernel - Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

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Date:   Wed, 30 Aug 2023 13:37:33 +0100
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>
Cc:     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>,
        Deepak Gupta <debug@...osinc.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        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 v4 03/36] arm64/gcs: Document the ABI for Guarded Control
 Stacks

The 08/24/2023 16:43, Catalin Marinas wrote:
> Is there a use-case for the unlocked configuration to allow disabling
> the GCS implicitly via a clone syscall?

how would you handle clone or clone3 without gcs specified?
(in the cases when clone creates a new thread with new stack)

(1) fail.
(2) allocate gcs.
(3) disable gcs.

the problem with (1) is that it requires changes to user code
(this only affects code outside the libc doing something special
since raw clone thread cannot call into the libc, all executing
code have to be tightly controlled. i don't know how common this
is, but i at least expect it in test code for system level tools
like debuggers, strace, valgrind, qemu-system, seccomp filters
etc. if it appears in actual use then that's a deployment issue
for distros: note that changing clone to clone3 is non-trivial:
it requires fallback logic and may have to deal with seccomp
filters).

problem with (2) is that the size policy and lifetime management
is in the kernel then. (since only special cases are affected i
guess that is ok, but i assumed we want to avoid this by moving
to clone3 and user managed gcs).

the problem with (3) is escaping the security measure, however
it only applies to very special threads that can always decide
to opt-in to gcs, so i don't see this as such a bad option and
at least bw compat with existing code. (in my threat model the
attacker cannot hijack clone syscalls as that seems stronger
than hijacking return addresses.)

so i guess the answer depends on how much headache failing
raw clone may cause and i don't know that.