linux-kernel - Re: [PATCH v4 17/36] arm64/mm: Handle GCS data aborts

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Message-ID: <752f9f06-69e3-4e60-94ca-49cacb1122ff@sirena.org.uk>
Date:   Wed, 16 Aug 2023 00:54:30 +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 17/36] arm64/mm: Handle GCS data aborts

On Fri, Aug 11, 2023 at 04:09:33PM +0100, Catalin Marinas wrote:
> On Mon, Aug 07, 2023 at 11:00:22PM +0100, Mark Brown wrote:

> > +		if (is_write_abort(esr) &&
> > +		    !(is_gcs_fault(esr) || is_el1_data_abort(esr)))
> > +			return VM_FAULT_BADACCESS;

> Related to my PIE permissions comment: when do we have a valid EL1 data
> write abort that's not a GCS fault? Does a faulting GCSSTTR set the
> ESR_ELx_GCS bit?

Yes, it should do.  The GCS instructions have access descriptors created
with CreateAccDescGCS() which results in the access being flagged as a
GCS access.

> > +	} else {
> > +		/*
> > +		 * GCS faults should never happen for pages that are
> > +		 * not part of a GCS and the operation being attempted
> > +		 * can never succeed.
> > +		 */
> > +		if (is_gcs_fault(esr))
> > +			return VM_FAULT_BADACCESS;

> If one does a GCS push/store to a non-GCS page, do we get a GCS fault or
> something else? I couldn't figure out from the engineering spec. If the
> hardware doesn't generate such exceptions, we might as well remove this
> 'else' branch. But maybe it does generate a GCS-specific fault as you
> added a similar check in is_invalid_el0_gcs_access().

Yes, see AddGCSRecord() and LoadCheckGCSRecord() - all GCS initiated
accesses need to be AccDescGCS so appropriate permissions enforcement
can happen and that's what causes the fault to be flagged as GCS.

> > @@ -595,6 +644,19 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
> >  	if (!vma)
> >  		goto lock_mmap;
> >  
> > +	/*
> > +	 * We get legitimate write faults for GCS pages from GCS
> > +	 * operations and from EL1 writes to EL0 pages but just plain

> What are the EL1 writes to the shadow stack? Would it not use
> copy_to_user_gcs()?

They should, yes - I'll reword the comment.

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