Re: [PATCH] KVM: LAPIC: Fix pv ipis use-before-initialization

[Paolo Bonzini]

On 20/11/18 02:39, Wanpeng Li wrote:
> Reported by syzkaller:
> 
>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000014
>  PGD 800000040410c067 P4D 800000040410c067 PUD 40410d067 PMD 0 
>  Oops: 0000 [#1] PREEMPT SMP PTI
>  CPU: 3 PID: 2567 Comm: poc Tainted: G           OE     4.19.0-rc5 #16
>  RIP: 0010:kvm_pv_send_ipi+0x94/0x350 [kvm]
>  Call Trace:
>   kvm_emulate_hypercall+0x3cc/0x700 [kvm]
>   handle_vmcall+0xe/0x10 [kvm_intel]
>   vmx_handle_exit+0xc1/0x11b0 [kvm_intel]
>   vcpu_enter_guest+0x9fb/0x1910 [kvm]
>   kvm_arch_vcpu_ioctl_run+0x35c/0x610 [kvm]
>   kvm_vcpu_ioctl+0x3e9/0x6d0 [kvm]
>   do_vfs_ioctl+0xa5/0x690
>   ksys_ioctl+0x6d/0x80
>   __x64_sys_ioctl+0x1a/0x20
>   do_syscall_64+0x83/0x6e0
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> The reason is that the apic map has not yet been initialized, the testcase 
> triggers pv_send_ipi interface by vmcall which results in kvm->arch.apic_map
> is dereferenced. This patch fixes it by checking whether or not apic map is 
> NULL and bailing out immediately if that is the case.
> 
> Fixes: 4180bf1b65 (KVM: X86: Implement "send IPI" hypercall)
> Reported-by: Wei Wu <ww9210@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Wei Wu <ww9210@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
>  arch/x86/kvm/lapic.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 3cd227f..09e3a12 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -576,6 +576,11 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
>  	rcu_read_lock();
>  	map = rcu_dereference(kvm->arch.apic_map);
>  
> +	if (unlikely(!map)) {
> +		count = -EOPNOTSUPP;
> +		goto out;
> +	}
> +
>  	if (min > map->max_apic_id)
>  		goto out;
>  	/* Bits above cluster_size are masked in the caller.  */
> 

Queued, thanks.

Paolo