linux-kernel - Re: kvm: WARNING in nested_vmx_merge_msr

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Message-ID: <20170301190806.GC17506@potion>
Date:   Wed, 1 Mar 2017 20:08:07 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steve Rutherford <srutherford@...gle.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        haozhong.zhang@...el.com, syzkaller <syzkaller@...glegroups.com>
Subject: Re: kvm: WARNING in nested_vmx_merge_msr_bitmap

2017-02-28 13:07-0800, Jim Mattson:
> I believe this happens when the VMCS12 MSR bitmap address is valid,
> but no device is configured to respond to the bus request. I agree
> that the warning should be removed. However, in this case, the VMCS12
> MSR bitmap should read as all 1s. The same is true everywhere that
> nested_get_page returns NULL. Reads should return all 1s and writes
> should be ignored.

Yes, we have already checked that the address is within physical limits
and page-aligned.  After that, any errors seem like undefined behavior.

Going to do a bare-metal check and send a patch, thanks.

> On Tue, Feb 28, 2017 at 11:06 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 7832 at arch/x86/kvm/vmx.c:9709
>>  warn_slowpath_null+0x2c/0x40 kernel/panic.c:583
>>  nested_vmx_merge_msr_bitmap arch/x86/kvm/vmx.c:9709 [inline]
>>  nested_get_vmcs12_pages+0xfb6/0x15c0 arch/x86/kvm/vmx.c:9640
>>  enter_vmx_non_root_mode arch/x86/kvm/vmx.c:10471 [inline]
>>  nested_vmx_run+0x6186/0xaab0 arch/x86/kvm/vmx.c:10561
>>
>> If I am reading this correctly, failure to map msr bitmap is trivially
>> trigger-able by guest (specify an invalid address?) and thus should
>> not be a warning. The case is already handled by disabling
>> acceleration. So can we remove the warning? or replace with pr_err?