linux-kernel - RE: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2ec9ecbfb13d422ab6cda355ff011c9f@AcuMS.aculab.com>
Date:   Thu, 16 Jun 2022 21:54:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'oliver.upton@...ux.dev'" <oliver.upton@...ux.dev>
CC:     Raghavendra Rao Ananta <rananta@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        "Ricardo Koller" <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        "Reiji Watanabe" <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        "Colton Lewis" <coltonlewis@...gle.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Andrew Jones" <drjones@...hat.com>
Subject: RE: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

From: oliver.upton@...ux.dev
> Sent: 16 June 2022 19:45

> 
> June 16, 2022 11:48 AM, "David Laight" <David.Laight@...lab.com> wrote:
> > No wonder I was confused.
> > It's not surprising the compiler optimises it all away.
> >
> > It doesn't seem right to be 'abusing' WRITE_ONCE() here.
> > Just adding barrier() should be enough and much more descriptive.
> 
> I had the same thought, although I do not believe barrier() is sufficient
> on its own. barrier_data() with a pointer to uc passed through
> is required to keep clang from eliminating the dead store.

A barrier() (full memory clobber) ought to be stronger than
the partial one than barrier_data() generates.

I can't quite decide whether you need a barrier() both sides
of the 'magic write'.
Plausibly the compiler could discard the on-stack data
after the barrier() and before the 'magic write'.

Certainly putting the 'magic write' inside a asm block
that has a memory clobber is a more correct solution.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)