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

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Date:   Tue, 12 Dec 2017 17:56:06 +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 17:40 GMT+08:00 David Hildenbrand <david@...hat.com>:
> 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.

How about this? https://lkml.org/lkml/2017/12/12/5

Regards,
Wanpeng Li