linux-kernel - Re: [PATCH v3 21/36] arm64/mm: Implement map_shadow

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Message-ID: <3a077ff3-8627-4337-9b4a-6a8828eda0e7@sirena.org.uk>
Date:   Mon, 7 Aug 2023 14:00:42 +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 v3 21/36] arm64/mm: Implement map_shadow_stack()

On Mon, Aug 07, 2023 at 11:20:58AM +0100, Szabolcs Nagy wrote:
> The 07/31/2023 14:43, Mark Brown wrote:
> > +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
> > +{
> > +	unsigned long alloc_size;
> > +	unsigned long __user *cap_ptr;
> > +	unsigned long cap_val;
> > +	int ret;
> > +
> > +	if (!system_supports_gcs())
> > +		return -EOPNOTSUPP;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (addr % 16)
> > +		return -EINVAL;

> mmap addr must be page aligned (and there is no align req on size).

> i'd expect similar api here.

That's not what the manual page or a quick check of the code suggest
that mmap() does, they say that the kernel just takes it as a hint and
chooses a nearby page boundary, though I didn't test.  I'm not sure why
I have that alignment check at all TBH, and to the extent it's needed I
could just be 8 - this level of code doesn't really care.

> > +	if (size == 16 || size % 16)
> > +		return -EINVAL;

> why %16 and not %8 ?

I don't think that's needed any more - there was some stuff in an
earlier version of the code but no longer.

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