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Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20200409085632.GB20713@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Apr 2020 03:13:30 -0700
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        kenny@panix.com, jeyu@kernel.org, rasmus.villemoes@prevas.dk,
        fenghua.yu@intel.com, xiaoyao.li@intel.com, thellstrom@vmware.com,
        tony.luck@intel.com, gregkh@linuxfoundation.org, jannh@google.com,
        keescook@chromium.org, David.Laight@aculab.com,
        dcovelli@vmware.com, mhiramat@kernel.org
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 9, 2020, at 1:56 AM, Peter Zijlstra <peterz@infradead.org> =
wrote:
>=20
> On Wed, Apr 08, 2020 at 06:15:48PM +0200, Paolo Bonzini wrote:
>> On 08/04/20 17:46, Christoph Hellwig wrote:
>=20
>>> Scanning all modules seems safer.  While we're at it - can be move =
the
>>> kvm bits using VMX to be always in the core kernel and just forbid
>>> modules from using those instructions entirely?
>>=20
>> I suppose we could use PVOPS-style patching for the more
>> performance-critical cases, but VMREAD/VMWRITE does not seem like a
>> particularly bad thing to allow modules and VMLAUNCH/VMRESUME have =
very
>> peculiar calling conventions around them.
>>=20
>> However, I wouldn't mind it if VMCLEAR/VMPTRLD and the associated =
kdump
>> cleanup code were moved to core kernel code.
>=20
> Speaking with my virt ignorance hat on, how impossible is it to =
provide
> generic/useful VMLAUNCH/VMRESUME wrappers?
>=20
> Because a lot of what happens around VMEXIT/VMENTER is very much like
> the userspace entry crud, as per that series from Thomas that fixes =
all
> that. And surely we don't need various broken copies of that in all =
the
> out-of-tree hypervisors.
>=20
> Also, I suppose if you have this, we no longer need to excempt CR2.

It depends on what you mean by =E2=80=9CVMLAUNCH/VMRESUME=E2=80=9D. If =
you only consider the
instructions themselves, as Sean did in vmx_vmenter() and vmx_vmexit(),
there is no problem. Even if you consider saving the general purpose
registers as done in __vmx_vcpu_run() - that=E2=80=99s relatively easy.

I think that anything that is greater than that, for instance
vmx_vcpu_run(), would require more thought and effort. KVM =
data-structures,
specifically kvm_vcpu and friends, would need to be broken into general =
and
KVM specific structures. I am really not sure that the end result would =
be
much better than using KVM user-space interfaces.

I can ask someone from the VMware hypervisor developers to provide =
VMware
point-of-view, but it would help to know when do you plan to make such a
change take, and whether there would be some transition stage. Adapting =
a
hypervisor to use different low-level interfaces would require quite =
some
development & testing effort.