linux-kernel - Re: [PATCH RESEND] KVM: X86: Fix load bad host fpu state

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Date:   Tue, 12 Dec 2017 05:51:26 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Rik van Riel <riel@...hat.com>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH RESEND] KVM: X86: Fix load bad host fpu state

2017-12-12 4:48 GMT+08:00 David Hildenbrand <david@...hat.com>:
> On 10.12.2017 22:44, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.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 if it is
>> uninitialized before KVM_RUN.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: stable@...r.kernel.org
>> Fixes: f775b13eedee (x86,kvm: move qemu/guest FPU switching out to vcpu_run)
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>>  arch/x86/kvm/x86.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index a92b22f..063a643 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7273,10 +7273,13 @@ 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;
>> +     struct fpu *fpu = &vcpu->arch.user_fpu;
>>       int r;
>>
>> -     fpu__initialize(fpu);
>> +     if (!fpu->initialized) {
>> +             fpstate_init(&fpu->state);
>> +             fpu->initialized = 1;
>> +     }
>
> Is there a chance of keeping using fpu__initialize() ? Duplicating the
> code is ugly.

There is a warning in fpu__initialize() which results in just
current->thread.fpu can take advantage of.

>
> E.g. can't we simply initialize that in kvm_load_guest_fpu?

We still miss to initialize qemu user FPU context for the above calltrace.

Regards,
Wanpeng Li