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

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Date:   Mon, 11 Dec 2017 21:48:03 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Rik van Riel <riel@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH RESEND] KVM: X86: Fix load bad host fpu state

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.

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


>  
>  	kvm_sigset_activate(vcpu);
>  
> 


-- 

Thanks,

David / dhildenb