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authorVitaly Kuznetsov <vkuznets@redhat.com>2021-03-31 14:41:29 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2021-04-01 05:14:19 -0400
commit77fcbe823f002ad18426545351fa2fb94f8d5e61 (patch)
tree23ce711101f3aab33c6fea0e336d947bf34ea49a /arch
parenta83829f56c7ce17d5d05370820e185d9a23d3090 (diff)
KVM: x86: Prevent 'hv_clock->system_time' from going negative in kvm_guest_time_update()
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kvm/x86.c19
1 files changed, 17 insertions, 2 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e1960eed921f..eca63625aee4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -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;
}