linux-kernel - Re: [PATCH 2/3] KVM: x86: Remove use of apicv_update

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: <2d86cce9-88c2-4b2f-a8a6-ee33d0e1c98d@redhat.com>
Date: Tue, 4 Feb 2025 18:58:38 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Naveen N Rao <naveen@...nel.org>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
 Vasant Hegde <vasant.hegde@....com>, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 2/3] KVM: x86: Remove use of apicv_update_lock when
 toggling guest debug state

On 2/4/25 18:51, Sean Christopherson wrote:
> On Tue, Feb 04, 2025, Naveen N Rao wrote:
>> On Mon, Feb 03, 2025 at 09:00:05PM -0500, Maxim Levitsky wrote:
>>> On Mon, 2025-02-03 at 22:33 +0530, Naveen N Rao (AMD) wrote:
>>>> apicv_update_lock is not required when querying the state of guest
>>>> debug in all the vcpus. Remove usage of the same, and switch to
>>>> kvm_set_or_clear_apicv_inhibit() helper to simplify the code.
>>>
>>> It might be worth to mention that the reason why the lock is not needed,
>>> is because kvm_vcpu_ioctl from which this function is called takes 'vcpu->mutex'
>>> and thus concurrent execution of this function is not really possible.
>>
>> Looking at this again, that looks to be a vcpu-specific lock, so I guess
>> it is possible for multiple vcpus to run this concurrently?
> 
> Correct.

And this patch is incorrect. Because there is a store and many loads, 
you have the typical race when two vCPUs set blockirq at the same time

	vcpu 0				vcpu 1
	---------------			--------------
	set vcpu0->guest_debug
					clear vcpu1->guest_debug
	read vcpu0->guest_debug
	read vcpu1->guest_debug	
	set inhibit
					read stale vcpu0->guest_debug
					read vcpu1->guest_debug
					clear inhibit

But since this is really a slow path, why even bother optimizing it?

Paolo