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    Subject[PATCH 5.5 257/367] KVM: x86: Revert "KVM: X86: Fix fpu state crash in kvm guest"
    Date
    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);

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