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[80.210.64.78]) by smtp.gmail.com with ESMTPSA id b36sm752130edd.81.2018.04.09.13.51.39 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 09 Apr 2018 13:51:40 -0700 (PDT) Date: Mon, 9 Apr 2018 22:51:39 +0200 From: Christoffer Dall To: Marc Zyngier Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Shannon Zhao , Mark Rutland Subject: Re: [PATCH] KVM: arm/arm64: Close VMID generation race Message-ID: <20180409205139.GH10904@cbox> References: <20180409170706.23541-1-marc.zyngier@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20180409170706.23541-1-marc.zyngier@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On Mon, Apr 09, 2018 at 06:07:06PM +0100, Marc Zyngier wrote: > Before entering the guest, we check whether our VMID is still > part of the current generation. In order to avoid taking a lock, > we start with checking that the generation is still current, and > only if not current do we take the lock, recheck, and update the > generation and VMID. > > This leaves open a small race: A vcpu can bump up the global > generation number as well as the VM's, but has not updated > the VMID itself yet. > > At that point another vcpu from the same VM comes in, checks > the generation (and finds it not needing anything), and jumps > into the guest. At this point, we end-up with two vcpus belonging > to the same VM running with two different VMIDs. Eventually, the > VMID used by the second vcpu will get reassigned, and things will > really go wrong... > > A simple solution would be to drop this initial check, and always take > the lock. This is likely to cause performance issues. A middle ground > is to convert the spinlock to a rwlock, and only take the read lock > on the fast path. If the check fails at that point, drop it and > acquire the write lock, rechecking the condition. > > This ensures that the above scenario doesn't occur. > > Reported-by: Mark Rutland > Signed-off-by: Marc Zyngier > --- > I haven't seen any reply from Shannon, so reposting this to > a slightly wider audience for feedback. > > virt/kvm/arm/arm.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c > index dba629c5f8ac..a4c1b76240df 100644 > --- a/virt/kvm/arm/arm.c > +++ b/virt/kvm/arm/arm.c > @@ -63,7 +63,7 @@ static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu); > static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1); > static u32 kvm_next_vmid; > static unsigned int kvm_vmid_bits __read_mostly; > -static DEFINE_SPINLOCK(kvm_vmid_lock); > +static DEFINE_RWLOCK(kvm_vmid_lock); > > static bool vgic_present; > > @@ -473,11 +473,16 @@ static void update_vttbr(struct kvm *kvm) > { > phys_addr_t pgd_phys; > u64 vmid; > + bool new_gen; > > - if (!need_new_vmid_gen(kvm)) > + read_lock(&kvm_vmid_lock); > + new_gen = need_new_vmid_gen(kvm); > + read_unlock(&kvm_vmid_lock); > + > + if (!new_gen) > return; > > - spin_lock(&kvm_vmid_lock); > + write_lock(&kvm_vmid_lock); > > /* > * We need to re-check the vmid_gen here to ensure that if another vcpu > @@ -485,7 +490,7 @@ static void update_vttbr(struct kvm *kvm) > * use the same vmid. > */ > if (!need_new_vmid_gen(kvm)) { > - spin_unlock(&kvm_vmid_lock); > + write_unlock(&kvm_vmid_lock); > return; > } > > @@ -519,7 +524,7 @@ static void update_vttbr(struct kvm *kvm) > vmid = ((u64)(kvm->arch.vmid) << VTTBR_VMID_SHIFT) & VTTBR_VMID_MASK(kvm_vmid_bits); > kvm->arch.vttbr = kvm_phys_to_vttbr(pgd_phys) | vmid; > > - spin_unlock(&kvm_vmid_lock); > + write_unlock(&kvm_vmid_lock); > } > > static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu) > -- > 2.14.2 > The above looks correct to me. I am wondering if something like the following would also work, which may be slightly more efficient, although I doubt the difference can be measured: Thanks, -Christoffer diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c index dba629c5f8ac..7ac869bcad21 100644 --- a/virt/kvm/arm/arm.c +++ b/virt/kvm/arm/arm.c @@ -458,7 +458,9 @@ void force_vm_exit(const cpumask_t *mask) */ static bool need_new_vmid_gen(struct kvm *kvm) { - return unlikely(kvm->arch.vmid_gen != atomic64_read(&kvm_vmid_gen)); + u64 current_vmid_gen = atomic64_read(&kvm_vmid_gen); + smp_rmb(); /* Orders read of kvm_vmid_gen and kvm->arch.vmid */ + return unlikely(kvm->arch.vmid_gen != current_vmid_gen); } /** @@ -508,10 +510,11 @@ static void update_vttbr(struct kvm *kvm) kvm_call_hyp(__kvm_flush_vm_context); } - kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen); kvm->arch.vmid = kvm_next_vmid; kvm_next_vmid++; kvm_next_vmid &= (1 << kvm_vmid_bits) - 1; + smp_wmb(); + kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen); /* update vttbr to be used with the new vmid */ pgd_phys = virt_to_phys(kvm->arch.pgd);