[PATCH v2] KVM: X86: Fix load bad host fpu state

[PATCH v2] KVM: X86: Fix load bad host fpu state

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@hotmail.com>

------------[ cut here ]------------
 Bad FPU state detected at kvm_put_guest_fpu+0xd8/0x2d0 [kvm], reinitializing FPU registers.
 WARNING: CPU: 1 PID: 4594 at arch/x86/mm/extable.c:103 ex_handler_fprestore+0x88/0x90
 CPU: 1 PID: 4594 Comm: qemu-system-x86 Tainted: G    B      OE    4.15.0-rc2+ #10
 RIP: 0010:ex_handler_fprestore+0x88/0x90
 Call Trace:
  fixup_exception+0x4e/0x60
  do_general_protection+0xff/0x270
  general_protection+0x22/0x30
 RIP: 0010:kvm_put_guest_fpu+0xd8/0x2d0 [kvm]
 RSP: 0018:ffff8803d5627810 EFLAGS: 00010246
  kvm_vcpu_reset+0x3b4/0x3c0 [kvm]
  kvm_apic_accept_events+0x1c0/0x240 [kvm]
  kvm_arch_vcpu_ioctl_run+0x1658/0x2fb0 [kvm]
  kvm_vcpu_ioctl+0x479/0x880 [kvm]
  do_vfs_ioctl+0x142/0x9a0
  SyS_ioctl+0x74/0x80
  do_syscall_64+0x15f/0x600

This can be reproduced by running any testcase in kvm-unit-tests since
the qemu userspace FPU context is not initialized, which results in the
init path from kvm_apic_accept_events() will load/put qemu userspace
FPU context w/o initialized. In addition, w/o this splatting we still
should initialize vcpu->arch.user_fpu instead of current->thread.fpu.
This patch fixes it by initializing qemu user FPU context during vCPU 
creation.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Fixes: f775b13eedee (x86,kvm: move qemu/guest FPU switching out to vcpu_run)
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * initialize user_fpu in fx_init

 arch/x86/kvm/x86.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5134a85..6a837fa 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7278,11 +7278,8 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 
 int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 {
-	struct fpu *fpu = &current->thread.fpu;
 	int r;
 
-	fpu__initialize(fpu);
-
 	kvm_sigset_activate(vcpu);
 
 	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
@@ -7709,6 +7706,7 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
 static void fx_init(struct kvm_vcpu *vcpu)
 {
 	fpstate_init(&vcpu->arch.guest_fpu.state);
+	fpstate_init(&vcpu->arch.user_fpu.state);
 	if (boot_cpu_has(X86_FEATURE_XSAVES))
 		vcpu->arch.guest_fpu.state.xsave.header.xcomp_bv =
 			host_xcr0 | XSTATE_COMPACTION_ENABLED;
-- 
2.7.4

Re: [PATCH v2] KVM: X86: Fix load bad host fpu state

[David Hildenbrand]

>  	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
> @@ -7709,6 +7706,7 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
>  static void fx_init(struct kvm_vcpu *vcpu)
>  {
>  	fpstate_init(&vcpu->arch.guest_fpu.state);
> +	fpstate_init(&vcpu->arch.user_fpu.state);

Won't at least one of them trigger the WARN in fpstate_init?

>  	if (boot_cpu_has(X86_FEATURE_XSAVES))
>  		vcpu->arch.guest_fpu.state.xsave.header.xcomp_bv =
>  			host_xcr0 | XSTATE_COMPACTION_ENABLED;
> 


-- 

Thanks,

David / dhildenb

Re: [PATCH v2] KVM: X86: Fix load bad host fpu state

[Wanpeng Li]

2017-12-13 17:59 GMT+08:00 David Hildenbrand <david@redhat.com>:
>
>>       if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
>> @@ -7709,6 +7706,7 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
>>  static void fx_init(struct kvm_vcpu *vcpu)
>>  {
>>       fpstate_init(&vcpu->arch.guest_fpu.state);
>> +     fpstate_init(&vcpu->arch.user_fpu.state);
>
> Won't at least one of them trigger the WARN in fpstate_init?

Nothing trigger the WARN during my testing on a Skylake box. And I
will not continue to push this one since there is an alternative
solution in kvm/queue.

Regards,
Wanpeng Li