linux-kernel - Re: [PATCH v3 01/16] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

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Date:   Thu, 11 Nov 2021 00:39:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, linux-csky@...r.kernel.org,
        linux-riscv@...ts.infradead.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Ingo Molnar <mingo@...hat.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Greentime Hu <green.hu@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Marc Zyngier <maz@...nel.org>, Nick Hu <nickhu@...estech.com>,
        Guo Ren <guoren@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v3 01/16] perf: Ensure perf_guest_cbs aren't reloaded
 between !NULL check and deref

On Wed, Nov 10, 2021, Paolo Bonzini wrote:
> On 11/4/21 15:18, Sean Christopherson wrote:
> > If I'm interpeting Paolo's suggestion
> > correctly, he's pointing out that oustanding stores to the function pointers in
> > @cbs need to complete before assigning a non-NULL pointer to perf_guest_cbs,
> > otherwise a perf event handler may see a valid pointer with half-baked callbacks.
> > 
> > I think smp_store_release() with a comment would be appropriate, assuming my
> > above interpretation is correct.
> > 
> 
> Yes, exactly.  It should even be rcu_assign_pointer(), matching the
> synchronize_rcu()

And perf_guest_cbs should be tagged __rcu and accessed accordingly.  Which is
effectively what this version (poorly) implemented with a homebrewed mix of
{READ,WRITE}_ONCE, lockdep(), and synchronize_rcu().

> in patch 1 (and the change can be done in patch 1, too).

Ya, the change needs to go into patch 1.