linux-kernel - Re: [PATCH] KVM: X86: Fix NULL deref in vcpu_scan

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Date:   Tue, 18 Dec 2018 22:03:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Fix NULL deref in vcpu_scan_ioapic

On 17/12/18 03:43, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Reported by syzkaller:
> 
>     CPU: 1 PID: 5962 Comm: syz-executor118 Not tainted 4.20.0-rc6+ #374
>     Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>     RIP: 0010:kvm_apic_hw_enabled arch/x86/kvm/lapic.h:169 [inline]
>     RIP: 0010:vcpu_scan_ioapic arch/x86/kvm/x86.c:7449 [inline]
>     RIP: 0010:vcpu_enter_guest arch/x86/kvm/x86.c:7602 [inline]
>     RIP: 0010:vcpu_run arch/x86/kvm/x86.c:7874 [inline]
>     RIP: 0010:kvm_arch_vcpu_ioctl_run+0x5296/0x7320 arch/x86/kvm/x86.c:8074
>     Call Trace:
> 	 kvm_vcpu_ioctl+0x5c8/0x1150 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2596
> 	 vfs_ioctl fs/ioctl.c:46 [inline]
> 	 file_ioctl fs/ioctl.c:509 [inline]
> 	 do_vfs_ioctl+0x1de/0x1790 fs/ioctl.c:696
> 	 ksys_ioctl+0xa9/0xd0 fs/ioctl.c:713
> 	 __do_sys_ioctl fs/ioctl.c:720 [inline]
> 	 __se_sys_ioctl fs/ioctl.c:718 [inline]
> 	 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
> 	 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> 	 entry_SYSCALL_64_after_hwframe+0x49/0xbe 
> 
> The reason is that the testcase writes hyperv synic HV_X64_MSR_SINT14 msr
> and triggers scan ioapic logic to load synic vectors into EOI exit bitmap.
> However, irqchip is not initialized by this simple testcase, ioapic/apic
> objects should not be accessed.
> 
> This patch fixes it by also considering whether or not apic is present.
> 
> Reported-by: syzbot+39810e6c400efadfef71@...kaller.appspotmail.com
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4f786fc..c7c1197 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7510,7 +7510,7 @@ void kvm_make_scan_ioapic_request(struct kvm *kvm)
>  
>  static void vcpu_scan_ioapic(struct kvm_vcpu *vcpu)
>  {
> -	if (!kvm_apic_hw_enabled(vcpu->arch.apic))
> +	if (!kvm_apic_present(vcpu))
>  		return;
>  
>  	bitmap_zero(vcpu->arch.ioapic_handled_vectors, 256);
> 

Queued, thanks.

Paolo