From: Oliver Upton <oupton@google.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
Paolo Bonzini <pbonzini@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
Jim Mattson <jmattson@google.com>,
David Matlack <dmatlack@google.com>,
Ricardo Koller <ricarkol@google.com>,
Jing Zhang <jingzhangos@google.com>,
Raghavendra Rao Anata <rananta@google.com>,
James Morse <james.morse@arm.com>,
Alexandru Elisei <Alexandru.Elisei@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
linux-arm-kernel@lists.infradead.org,
Andrew Jones <drjones@redhat.com>, Will Deacon <will@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v7 1/6] KVM: x86: Fix potential race in KVM_GET_CLOCK
Date: Fri, 20 Aug 2021 11:22:38 -0700 [thread overview]
Message-ID: <CAOQ_Qsin6YTUdbGaq3GChFWq_fzmXrKOUTQk7FarqL0b5GqC4g@mail.gmail.com> (raw)
In-Reply-To: <20210819182422.GA25923@fuller.cnet>
On Thu, Aug 19, 2021 at 11:43 AM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Mon, Aug 16, 2021 at 12:11:25AM +0000, Oliver Upton wrote:
> > Sean noticed that KVM_GET_CLOCK was checking kvm_arch.use_master_clock
> > outside of the pvclock sync lock. This is problematic, as the clock
> > value written to the user may or may not actually correspond to a stable
> > TSC.
> >
> > Fix the race by populating the entire kvm_clock_data structure behind
> > the pvclock_gtod_sync_lock.
>
> Oliver,
>
> Can you please describe the race in more detail?
>
> Is it about host TSC going unstable VS parallel KVM_GET_CLOCK ?
>
Yeah, pretty much any event that causes us to set use_master_clock =
false could interleave with the KVM_GET_CLOCK ioctl. A guest could
kick its TSCs out of sync, for example, to cause this too. AFAICT, KVM
serializes the write side (pvclock_update_vm_gtod_copy()) with
pvclock_gtod_sync_lock, as it should.
--
Thanks,
Oliver
next prev parent reply other threads:[~2021-08-20 18:24 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-16 0:11 [PATCH v7 0/6] KVM: x86: Add idempotent controls for migrating system counter state Oliver Upton
2021-08-16 0:11 ` [PATCH v7 1/6] KVM: x86: Fix potential race in KVM_GET_CLOCK Oliver Upton
2021-08-19 18:24 ` Marcelo Tosatti
2021-08-20 18:22 ` Oliver Upton [this message]
2021-08-16 0:11 ` [PATCH v7 2/6] KVM: x86: Create helper methods for KVM_{GET,SET}_CLOCK ioctls Oliver Upton
2021-08-16 0:11 ` [PATCH v7 3/6] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK Oliver Upton
2021-08-20 12:46 ` Marcelo Tosatti
2021-09-24 8:30 ` Paolo Bonzini
2021-08-16 0:11 ` [PATCH v7 4/6] KVM: x86: Take the pvclock sync lock behind the tsc_write_lock Oliver Upton
2021-09-02 19:22 ` Sean Christopherson
2021-08-16 0:11 ` [PATCH v7 5/6] KVM: x86: Refactor tsc synchronization code Oliver Upton
2021-09-02 19:21 ` Sean Christopherson
2021-09-02 19:41 ` Oliver Upton
2021-09-24 9:28 ` Paolo Bonzini
2021-08-16 0:11 ` [PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace Oliver Upton
2021-08-23 20:56 ` Oliver Upton
2021-08-26 12:48 ` Marcelo Tosatti
2021-08-26 20:27 ` Oliver Upton
2021-09-02 19:23 ` [PATCH v7 0/6] KVM: x86: Add idempotent controls for migrating system counter state Sean Christopherson
2021-09-02 19:45 ` Oliver Upton
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