linux-kernel - Re: [PATCH 22/35] arm64/mm: Implement map_shadow

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Message-ID: <8ea03eda-e630-40c5-acc1-a63f5c5b3102@sirena.org.uk>
Date:   Tue, 18 Jul 2023 14:55:07 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Szabolcs Nagy <Szabolcs.Nagy@....com>
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>,
        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 22/35] arm64/mm: Implement map_shadow_stack()

On Tue, Jul 18, 2023 at 10:10:04AM +0100, Szabolcs Nagy wrote:

>   uint64_t *p = map_shadow_stack(0, N*8, 0);

> i'd expect p[N-1] to be the end token and p[N-2] to be the cap token,
> not p[PAGE_ALIGN(N*8)/8-2].

Yes, that probably would be more helpful.

> if we allow misalligned size here (and in munmap) then i think it's
> better to not page align.  size%8!=0 || size<16 can be an error.

Honestly I'd be a lot happier to just not allow misalignment but that
raises the issue with binaries randomly not working when moved to a
kernel with a different page size.  I'll have a think but possibly the
safest thing would be requiring a multiple of 4K then rounding up to our
actual page size.

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