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Thu, 16 Sep 2021 11:15:44 -0700 (PDT) Date: Thu, 16 Sep 2021 18:15:34 +0000 In-Reply-To: <20210916181538.968978-1-oupton@google.com> Message-Id: <20210916181538.968978-4-oupton@google.com> Mime-Version: 1.0 References: <20210916181538.968978-1-oupton@google.com> X-Mailer: git-send-email 2.33.0.464.g1972c5931b-goog Subject: [PATCH v8 3/7] KVM: x86: Fix potential race in KVM_GET_CLOCK From: Oliver Upton To: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu Cc: Catalin Marinas , Will Deacon , Marc Zyngier , Peter Shier , Sean Christopherson , David Matlack , Paolo Bonzini , linux-arm-kernel@lists.infradead.org, Jim Mattson X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu 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. Suggested-by: Sean Christopherson Signed-off-by: Oliver Upton --- arch/x86/kvm/x86.c | 36 +++++++++++++++++++++++------------- 1 file changed, 23 insertions(+), 13 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index c910cf31958f..523c4e5c109f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2793,19 +2793,20 @@ static void kvm_update_masterclock(struct kvm *kvm) kvm_end_pvclock_update(kvm); } -u64 get_kvmclock_ns(struct kvm *kvm) +static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data) { struct kvm_arch *ka = &kvm->arch; struct pvclock_vcpu_time_info hv_clock; unsigned long flags; - u64 ret; spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags); if (!ka->use_master_clock) { spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags); - return get_kvmclock_base_ns() + ka->kvmclock_offset; + data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset; + return; } + data->flags |= KVM_CLOCK_TSC_STABLE; hv_clock.tsc_timestamp = ka->master_cycle_now; hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags); @@ -2817,13 +2818,26 @@ u64 get_kvmclock_ns(struct kvm *kvm) kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL, &hv_clock.tsc_shift, &hv_clock.tsc_to_system_mul); - ret = __pvclock_read_cycles(&hv_clock, rdtsc()); - } else - ret = get_kvmclock_base_ns() + ka->kvmclock_offset; + data->clock = __pvclock_read_cycles(&hv_clock, rdtsc()); + } else { + data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset; + } put_cpu(); +} - return ret; +u64 get_kvmclock_ns(struct kvm *kvm) +{ + struct kvm_clock_data data; + + /* + * Zero flags as it's accessed RMW, leave everything else uninitialized + * as clock is always written and no other fields are consumed. + */ + data.flags = 0; + + get_kvmclock(kvm, &data); + return data.clock; } static void kvm_setup_pvclock_page(struct kvm_vcpu *v, @@ -5832,13 +5846,9 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state) static int kvm_vm_ioctl_get_clock(struct kvm *kvm, void __user *argp) { struct kvm_clock_data data; - u64 now_ns; - - now_ns = get_kvmclock_ns(kvm); - user_ns.clock = now_ns; - user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0; - memset(&user_ns.pad, 0, sizeof(user_ns.pad)); + memset(&data, 0, sizeof(data)); + get_kvmclock(kvm, &data); if (copy_to_user(argp, &data, sizeof(data))) return -EFAULT; -- 2.33.0.309.g3052b89438-goog _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm