linux-kernel - Re: [PATCH v4 09/36] arm64/mm: Allocate PIE slots for EL0 guarded control stack

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Date:   Wed, 16 Aug 2023 00:21:49 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
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>,
        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>,
        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 09/36] arm64/mm: Allocate PIE slots for EL0 guarded
 control stack

On Fri, Aug 11, 2023 at 03:23:12PM +0100, Catalin Marinas wrote:

> >  #define PIE_E1	( \
> > +	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS),           PIE_RW)      | \
> > +	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO),        PIE_R)      | \

> Had some thoughts on this. Why do we need the EL1 GCS attributes to map
> to RW? The instructions we'd use to write the shadow stack are the GCS
> 'T' variants that run as user already.

> The only instructions we have in the kernel that would run as EL1 on a
> user address are the exclusives (futex code or the old deprecated
> emulation but we don't care about them in this context). So I wonder
> whether the kernel PIE entry could simply be PIE_NONE_O. Would this be
> too restrictive for future uses? Given the coherency between a GCS
> access and a standard data access, we may want to restrict it now until
> we have a use-case.

Good point.  I remember I originally wrote that before I checked into
how things like copying pages for ptrace worked but they don't keep
the GCSness of the page so they're fine.

I don't think we need to worry about future uses since these are slots
reserved for GCS use, if we need a different value later

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