linux-kernel - Re: [PATCH 5/5] perf: Correct perf sampling with guest VMs

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: <ZtmOENs5qveMH920@J2N7QTR9R3>
Date: Thu, 5 Sep 2024 11:55:12 +0100
From: Mark Rutland <mark.rutland@....com>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, Oliver Upton <oliver.upton@...ux.dev>,
	Sean Christopherson <seanjc@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Will Deacon <will@...nel.org>, Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Naveen N Rao <naveen@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 5/5] perf: Correct perf sampling with guest VMs

On Wed, Sep 04, 2024 at 08:41:33PM +0000, Colton Lewis wrote:
> Previously any PMU overflow interrupt that fired while a VCPU was
> loaded was recorded as a guest event whether it truly was or not. This
> resulted in nonsense perf recordings that did not honor
> perf_event_attr.exclude_guest and recorded guest IPs where it should
> have recorded host IPs.
> 
> Reorganize that plumbing to record perf events correctly even when
> VCPUs are loaded.

It'd be good if we could make that last bit a little more explicit,
e.g.

  Rework the sampling logic to only record guest samples for events with
  exclude_guest clear. This way any host-only events with exclude_guest
  set will never see unexpected guest samples. The behaviour of events
  with exclude_guest clear is unchanged.

[...]

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4384f6c49930..e1a66c9c3773 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6915,13 +6915,26 @@ void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
>  #endif
>  
> -unsigned long perf_misc_flags(unsigned long pt_regs *regs)
> +static bool is_guest_event(struct perf_event *event)
>  {
> +	return !event->attr.exclude_guest && perf_guest_state();
> +}

Could we name this something like "should_sample_guest()"? Calling this
"is_guest_event()" makes it should like it's checking a static property
of the event (and not other conditions like perf_guest_state()).

Otherwise this all looks reasonable to me, modulo Ingo's comments. I'll
happily test a v2 once those have been addressed.

Mark.