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

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Date:   Tue, 12 Dec 2017 10:40:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Wanpeng Li <kernellwp@...il.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

On 11.12.2017 22:51, Wanpeng Li wrote:
> 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.

Wonder if it would make more sense to

a) drop the WARN
b) introduce a new variant without the WARN (the existing one can call this)

Manual initialization looks ugly.

> 
>>
>> 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.

As load will happen before store we should be fine. But I guess it will
result in the same problem.

> 
> Regards,
> Wanpeng Li
> 


-- 

Thanks,

David / dhildenb