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KVM: x86: Prevent 'hv_clock->system_time' from going negative in kvm_guest_time_update()

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When guest time is reset with KVM_SET_CLOCK(0), it is possible for
'hv_clock->system_time' to become a small negative number. This happens
because in KVM_SET_CLOCK handling we set 'kvm->arch.kvmclock_offset' based
on get_kvmclock_ns(kvm) but when KVM_REQ_CLOCK_UPDATE is handled,
kvm_guest_time_update() does (masterclock in use case):

hv_clock.system_time = ka->master_kernel_ns + v->kvm->arch.kvmclock_offset;

And 'master_kernel_ns' represents the last time when masterclock
got updated, it can precede KVM_SET_CLOCK() call. Normally, this is not a
problem, the difference is very small, e.g. I'm observing
hv_clock.system_time = -70 ns. The issue comes from the fact that
'hv_clock.system_time' is stored as unsigned and 'system_time / 100' in
compute_tsc_page_parameters() becomes a very big number.

Use 'master_kernel_ns' instead of get_kvmclock_ns() when masterclock is in
use and get_kvmclock_base_ns() when it's not to prevent 'system_time' from
going negative.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210331124130.337992-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2021-03-31 14:41:29 +02:00 committed by Paolo Bonzini
parent a83829f56c
commit 77fcbe823f
1 changed files with 17 additions and 2 deletions
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19
arch/x86/kvm/x86.c
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@ -5727,6 +5727,7 @@ set_pit2_out:
}
#endif
case KVM_SET_CLOCK: {
struct kvm_arch *ka = &kvm->arch;
struct kvm_clock_data user_ns;
u64 now_ns;
@ -5745,8 +5746,22 @@ set_pit2_out:
* pvclock_update_vm_gtod_copy().
*/
kvm_gen_update_masterclock(kvm);
now_ns = get_kvmclock_ns(kvm);
kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
/*
* This pairs with kvm_guest_time_update(): when masterclock is
* in use, we use master_kernel_ns + kvmclock_offset to set
* unsigned 'system_time' so if we use get_kvmclock_ns() (which
* is slightly ahead) here we risk going negative on unsigned
* 'system_time' when 'user_ns.clock' is very small.
*/
spin_lock_irq(&ka->pvclock_gtod_sync_lock);
if (kvm->arch.use_master_clock)
now_ns = ka->master_kernel_ns;
else
now_ns = get_kvmclock_base_ns();
ka->kvmclock_offset = user_ns.clock - now_ns;
spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
break;
}