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

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Message-ID: <ZsdzKrTbrolW0lHn@arm.com>
Date: Thu, 22 Aug 2024 18:19:38 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: 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>,
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Subject: Re: [PATCH v11 19/39] arm64/mm: Handle GCS data aborts

On Thu, Aug 22, 2024 at 05:44:19PM +0100, Mark Brown wrote:
> 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);

We do need the write abort check but not unconditionally, only if to a
GCS page (you can have other genuine write aborts).

-- 
Catalin