Commit 55c0cefb authored by Oliver Upton's avatar Oliver Upton Committed by Paolo Bonzini

KVM: x86: Fix potential race in KVM_GET_CLOCK

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: default avatarSean Christopherson <seanjc@google.com>
Signed-off-by: default avatarOliver Upton <oupton@google.com>
Message-Id: <20210916181538.968978-4-oupton@google.com>
Reviewed-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 45e6c2fa
......@@ -2781,19 +2781,20 @@ 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);
......@@ -2805,13 +2806,26 @@ 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,
......@@ -5820,13 +5834,9 @@ 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;
......
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