linux-kernel - Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

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Date:   Sat, 28 Aug 2021 21:44:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        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>,
        "H. Peter Anvin" <hpa@...or.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-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.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>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded
 between !NULL check and deref

On Fri, Aug 27, 2021 at 05:35:46PM -0700, Sean Christopherson wrote:

> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 2d510ad750ed..6b0405e578c1 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1237,6 +1237,14 @@ extern void perf_event_bpf_event(struct bpf_prog *prog,
>  				 u16 flags);
>  
>  extern struct perf_guest_info_callbacks *perf_guest_cbs;
> +static inline struct perf_guest_info_callbacks *perf_get_guest_cbs(void)
> +{
> +	/* Reg/unreg perf_guest_cbs waits for readers via synchronize_rcu(). */
> +	lockdep_assert_preemption_disabled();
> +
> +	/* Prevent reloading between a !NULL check and dereferences. */
> +	return READ_ONCE(perf_guest_cbs);
> +}

Nice..

>  extern int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *callbacks);
>  extern int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *callbacks);
>  
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 464917096e73..2126f6327321 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6491,14 +6491,19 @@ struct perf_guest_info_callbacks *perf_guest_cbs;
>  
>  int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  {
> -	perf_guest_cbs = cbs;
> +	if (WARN_ON_ONCE(perf_guest_cbs))
> +		return -EBUSY;
> +
> +	WRITE_ONCE(perf_guest_cbs, cbs);
> +	synchronize_rcu();

You're waiting for all NULL users to go away? :-) IOW, we can do without
this synchronize_rcu() call.

>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
>  
>  int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  {
> -	perf_guest_cbs = NULL;

	if (WARN_ON_ONCE(perf_guest_cbs != cbs))
		return -EBUSY;

?

> +	WRITE_ONCE(perf_guest_cbs, NULL);
> +	synchronize_rcu();
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);

Yes, this ought to work fine.