linux-kernel - Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

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 for Android: free password hash cracker in your pocket

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

Message-ID: <20251202100311.GB2458571@noisy.programming.kicks-ass.net>
Date: Tue, 2 Dec 2025 11:03:11 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Fernand Sieber <sieberf@...zon.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com,
	Jan H. Schönherr <jschoenh@...zon.de>,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	dwmw@...zon.co.uk, hborghor@...zon.de, nh-open-source@...zon.com,
	abusse@...zon.de, nsaenz@...zon.com, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

On Mon, Dec 01, 2025 at 04:23:57PM +0200, Fernand Sieber wrote:
>  arch/x86/kvm/pmu.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 487ad19a236e..547512028e24 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -225,6 +225,19 @@ static u64 get_sample_period(struct kvm_pmc *pmc, u64 counter_value)
>  {
>  	u64 sample_period = (-counter_value) & pmc_bitmask(pmc);
>  
> +	/*
> +	 * A sample_period of 1 might get mistaken by perf for a BTS event, see
> +	 * intel_pmu_has_bts_period(). This would prevent re-arming the counter
> +	 * via pmc_resume_counter(), followed by the accidental creation of an
> +	 * actual BTS event, which we do not want.
> +	 *
> +	 * Avoid this by bumping the sampling period. Note, that we do not lose
> +	 * any precision, because the same quirk happens later anyway (for
> +	 * different reasons) in x86_perf_event_set_period().
> +	 */
> +	if (sample_period == 1)
> +		sample_period = 2;
> +
>  	if (!sample_period)
>  		sample_period = pmc_bitmask(pmc) + 1;
>  	return sample_period;

Oh gawd, I so hate this kvm code. It is so ludicrously bad. The way it
keeps recreating counters is just stupid. And then they complain it
sucks, it does :-(

Anyway, yes this is terrible. Let me try and untangle all this, see if
there's a saner solution.