linux-kernel - Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc

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: <1523564672.32594.35.camel@amazon.de>
Date:   Thu, 12 Apr 2018 20:24:32 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "jmattson@...gle.com" <jmattson@...gle.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running
 an L2 guest

On Thu, 2018-04-12 at 22:21 +0200, Paolo Bonzini wrote:
> On 12/04/2018 19:21, Raslan, KarimAllah wrote:
> > 
> > Now looking further at the code, it seems that everywhere in the code
> > tsc_offset is treated as the L01 TSC_OFFSET.
> > 
> > Like here:
> > 
> >         if (vmcs12->cpu_based_vm_exec_control &
> > CPU_BASED_USE_TSC_OFFSETING)
> >                 vmcs_write64(TSC_OFFSET,
> >                         vcpu->arch.tsc_offset + vmcs12->tsc_offset);
> > 
> > and here:
> > 
> >         vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> > 
> > and here:
> > 
> > u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> > {
> >         return vcpu->arch.tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> > }
> > EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
> > 
> > ... would not it be simpler and more inline with the current code to
> > just do what I did above + remove the "+ l1_tsc_offset" + probably
> > document tsc_offset ?
> 
> Problem is, I don't think it's correct. :)  A good start would be to try
> disabling MSR_IA32_TSC interception in KVM, prepare a kvm-unit-tests
> test that reads the MSR, and see if you get the host or guest TSC...

I actually just submitted a patch with your original suggestion
(I hope)  because I realized that adjust tsc was still using the wrong
tsc_offset anyway :)

> 
> Paolo
> 
Amazon Development Center Germany GmbH
Berlin - Dresden - Aachen
main office: Krausenstr. 38, 10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B