linux-kernel - Re: [PATCH RESEND] KVM: X86: Fix stack-out-of-bounds read in write

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Message-ID: <CANRm+CwLmY7R6oB=Ah-+sof7XW9eo2kkv5kmrbjxJJfisnovog@mail.gmail.com>
Date:   Wed, 13 Dec 2017 11:05:31 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH RESEND] KVM: X86: Fix stack-out-of-bounds read in write_mmio

2017-12-13 0:07 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 12/12/2017 09:57, Wanpeng Li wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index bc5d853..51e7932 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4690,7 +4690,10 @@ static int write_emulate(struct kvm_vcpu *vcpu, gpa_t gpa,
>>
>>  static int write_mmio(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes, void *val)
>>  {
>> -     trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, *(u64 *)val);
>> +     u64 data = 0;
>> +
>> +     memcpy(&data, val, min(8, bytes));
>> +     trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, data);
>>       return vcpu_mmio_write(vcpu, gpa, bytes, val);
>>  }
>>
>>
>
> Please do the memcpy in TRACE_EVENT(kvm_mmio)'s TP_fast_assign block.
> That is done only when the trace event is active.

I still can observe the stack out-of-bounds read warning if keep *(u64
*)val as the parameter since it has already been dereferenced. So
maybe we should change the parameter of trace_kvm_mmio() to void *val,
however, I'm not sure whether it will break the tracepoint ABI which
this article https://lwn.net/Articles/734039/ "Workload analysis with
tracing" section pointed out.

Regards,
Wanpeng Li