From: Sean Christopherson sean.j.christopherson@intel.com
commit 2620fe268e80d667a94553cd37a94ccaa2cb8c83 upstream.
Reload the current thread's FPU state, which contains the guest's FPU state, to the CPU registers if necessary during vcpu_enter_guest(). TIF_NEED_FPU_LOAD can be set any time control is transferred out of KVM, e.g. if I/O is triggered during a KVM call to get_user_pages() or if a softirq occurs while KVM is scheduled in.
Moving the handling of TIF_NEED_FPU_LOAD from vcpu_enter_guest() to kvm_arch_vcpu_load(), effectively kvm_sched_in(), papered over a bug where kvm_put_guest_fpu() failed to account for TIF_NEED_FPU_LOAD. The easiest way to the kvm_put_guest_fpu() bug was to run with involuntary preemption enable, thus handling TIF_NEED_FPU_LOAD during kvm_sched_in() made the bug go away. But, removing the handling in vcpu_enter_guest() exposed KVM to the rare case of a softirq triggering kernel_fpu_begin() between vcpu_load() and vcpu_enter_guest().
Now that kvm_{load,put}_guest_fpu() correctly handle TIF_NEED_FPU_LOAD, revert the commit to both restore the vcpu_enter_guest() behavior and eliminate the superfluous switch_fpu_return() in kvm_arch_vcpu_load().
Note, leaving the handling in kvm_arch_vcpu_load() isn't wrong per se, but it is unnecessary, and most critically, makes it extremely difficult to find bugs such as the kvm_put_guest_fpu() issue due to shrinking the window where a softirq can corrupt state.
A sample trace triggered by warning if TIF_NEED_FPU_LOAD is set while vcpu state is loaded:
<IRQ> gcmaes_crypt_by_sg.constprop.12+0x26e/0x660 ? 0xffffffffc024547d ? __qdisc_run+0x83/0x510 ? __dev_queue_xmit+0x45e/0x990 ? ip_finish_output2+0x1a8/0x570 ? fib4_rule_action+0x61/0x70 ? fib4_rule_action+0x70/0x70 ? fib_rules_lookup+0x13f/0x1c0 ? helper_rfc4106_decrypt+0x82/0xa0 ? crypto_aead_decrypt+0x40/0x70 ? crypto_aead_decrypt+0x40/0x70 ? crypto_aead_decrypt+0x40/0x70 ? esp_output_tail+0x8f4/0xa5a [esp4] ? skb_ext_add+0xd3/0x170 ? xfrm_input+0x7a6/0x12c0 ? xfrm4_rcv_encap+0xae/0xd0 ? xfrm4_transport_finish+0x200/0x200 ? udp_queue_rcv_one_skb+0x1ba/0x460 ? udp_unicast_rcv_skb.isra.63+0x72/0x90 ? __udp4_lib_rcv+0x51b/0xb00 ? ip_protocol_deliver_rcu+0xd2/0x1c0 ? ip_local_deliver_finish+0x44/0x50 ? ip_local_deliver+0xe0/0xf0 ? ip_protocol_deliver_rcu+0x1c0/0x1c0 ? ip_rcv+0xbc/0xd0 ? ip_rcv_finish_core.isra.19+0x380/0x380 ? __netif_receive_skb_one_core+0x7e/0x90 ? netif_receive_skb_internal+0x3d/0xb0 ? napi_gro_receive+0xed/0x150 ? 0xffffffffc0243c77 ? net_rx_action+0x149/0x3b0 ? __do_softirq+0xe4/0x2f8 ? handle_irq_event_percpu+0x6a/0x80 ? irq_exit+0xe6/0xf0 ? do_IRQ+0x7f/0xd0 ? common_interrupt+0xf/0xf </IRQ> ? irq_entries_start+0x20/0x660 ? vmx_get_interrupt_shadow+0x2f0/0x710 [kvm_intel] ? kvm_set_msr_common+0xfc7/0x2380 [kvm] ? recalibrate_cpu_khz+0x10/0x10 ? ktime_get+0x3a/0xa0 ? kvm_arch_vcpu_ioctl_run+0x107/0x560 [kvm] ? kvm_init+0x6bf/0xd00 [kvm] ? __seccomp_filter+0x7a/0x680 ? do_vfs_ioctl+0xa4/0x630 ? security_file_ioctl+0x32/0x50 ? ksys_ioctl+0x60/0x90 ? __x64_sys_ioctl+0x16/0x20 ? do_syscall_64+0x5f/0x1a0 ? entry_SYSCALL_64_after_hwframe+0x44/0xa9 ---[ end trace 9564a1ccad733a90 ]---
This reverts commit e751732486eb3f159089a64d1901992b1357e7cc.
Fixes: e751732486eb3 ("KVM: X86: Fix fpu state crash in kvm guest") Reported-by: Derek Yerger derek@djy.llc Reported-by: kernel@najdan.com Cc: Wanpeng Li wanpengli@tencent.com Cc: Thomas Lambertz mail@thomaslambertz.de Cc: Rik van Riel riel@surriel.com Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Borislav Petkov bp@suse.de Cc: Dave Hansen dave.hansen@intel.com Cc: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/kvm/x86.c | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-)
--- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3496,10 +3496,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu
kvm_x86_ops->vcpu_load(vcpu, cpu);
- fpregs_assert_state_consistent(); - if (test_thread_flag(TIF_NEED_FPU_LOAD)) - switch_fpu_return(); - /* Apply any externally detected TSC adjustments (due to suspend) */ if (unlikely(vcpu->arch.tsc_offset_adjustment)) { adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment); @@ -8244,8 +8240,9 @@ static int vcpu_enter_guest(struct kvm_v trace_kvm_entry(vcpu->vcpu_id); guest_enter_irqoff();
- /* The preempt notifier should have taken care of the FPU already. */ - WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD)); + fpregs_assert_state_consistent(); + if (test_thread_flag(TIF_NEED_FPU_LOAD)) + switch_fpu_return();
if (unlikely(vcpu->arch.switch_db_regs)) { set_debugreg(0, 7);