linux-kernel - Re: [PATCH v11 19/39] arm64/mm: Handle GCS data aborts

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Message-ID: <Zsdq4ymaW0vQffH_@finisterre.sirena.org.uk>
Date: Thu, 22 Aug 2024 17:44:19 +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>,
	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>, Kees Cook <kees@...nel.org>,
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	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
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	Yury Khrustalev <yury.khrustalev@....com>,
	Wilco Dijkstra <wilco.dijkstra@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v11 19/39] arm64/mm: Handle GCS data aborts

On Thu, Aug 22, 2024 at 05:12:30PM +0100, Catalin Marinas wrote:
> On Thu, Aug 22, 2024 at 02:15:22AM +0100, Mark Brown wrote:

> > +static bool is_invalid_gcs_access(struct vm_area_struct *vma, u64 esr)

> > +	} else if (unlikely(vma->vm_flags & VM_SHADOW_STACK)) {
> > +		/* Only GCS operations can write to a GCS page */
> > +		return is_write_abort(esr);
> > +	}

> I don't think that's right. The ESR on this path may not even indicate a
> data abort and ESR.WnR bit check wouldn't make sense.

> I presume we want to avoid an infinite loop on a (writeable) GCS page
> when the user does a normal STR but the CPU raises a permission fault. I
> think this function needs to just return false if !esr_is_data_abort().

Yes, that should check for a data abort.  I think I'd formed the
impression that is_write_abort() included that check somehow.  As you
say it's to avoid spinning trying to resolve a permission fault for a
write (non-GCS reads to a GCS page are valid), I do think we need the 
is_write_abort() since non-GCS reads are valid so something like:

	if (!esr_is_data_abort(esr))
		return false;

	return is_write_abort(esr);



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