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    Subject[PATCH 3.16 291/305] KVM: X86: Fix NULL deref in vcpu_scan_ioapic
    3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Wanpeng Li <wanpengli@tencent.com>

    commit dcbd3e49c2f0b2c2d8a321507ff8f3de4af76d7c upstream.

    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@syzkaller.appspotmail.com
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Radim Krčmář <rkrcmar@redhat.com>
    Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/x86/kvm/x86.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -6054,7 +6054,7 @@ static void vcpu_scan_ioapic(struct kvm_
    u64 eoi_exit_bitmap[4];
    u32 tmr[8];

    - if (!kvm_apic_hw_enabled(vcpu->arch.apic))
    + if (!kvm_apic_present(vcpu))
    return;

    memset(eoi_exit_bitmap, 0, 32);
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