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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: <20200407205042.GT2452@worktop.programming.kicks-ass.net>
Date:   Tue, 7 Apr 2020 14:22:11 -0700
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, hch@infradead.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        mingo <mingo@redhat.com>, bp <bp@alien8.de>, hpa@zytor.com,
        x86 <x86@kernel.org>, "Kenneth R. Crudup" <kenny@panix.com>,
        Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Tony Luck <tony.luck@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jannh@google.com, keescook@chromium.org, David.Laight@aculab.com,
        Doug Covelli <dcovelli@vmware.com>, mhiramat@kernel.org
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> On Apr 7, 2020, at 1:50 PM, Peter Zijlstra <peterz@infradead.org> =
wrote:
>=20
> On Tue, Apr 07, 2020 at 01:27:45PM -0700, Nadav Amit wrote:
>>> On Apr 7, 2020, at 12:38 PM, Peter Zijlstra <peterz@infradead.org> =
wrote:
>>>=20
>>> On Tue, Apr 07, 2020 at 11:55:21AM -0700, Nadav Amit wrote:
>>>>> On Apr 7, 2020, at 4:02 AM, Peter Zijlstra <peterz@infradead.org> =
wrote:
>>>>>=20
>>>>> Since we now have infrastructure to analyze module text, disallow
>>>>> modules that write to CRn and DRn registers.
>>>>=20
>>>> Assuming the kernel is built without CONFIG_PARAVIRT, what is the =
right way
>>>> for out-of-tree modules to write to CRs? Let=E2=80=99s say CR2?
>>>=20
>>> Most of them there is no real justification for ever writing to. CR2 =
I
>>> suppose we can have an exception for given a sane rationale for why
>>> you'd need to rewrite the fault address.
>>=20
>> For the same reason that KVM writes to CR2 - to restore CR2 before =
entering
>> a guest, since CR2 not architecturally loaded from the VMCS. I =
suspect there
>> are additional use-cases which are not covered by the kernel =
interfaces.
>=20
> So I'm not much of a virt guy (clearly), and *groan*, that's horrible.
> I'll go make an exception for CR2.

Clearly you are not a virt guy if you think that this is the horrible =
part
in x86 virtualization ;-)

Anyhow, I do not think it is the only use-case which is not covered by =
your
patches (even considering CRs/DRs alone). For example, there is no =
kernel
function to turn on CR4.VMXE, which is required to run hypervisors on =
x86.

I think a thorough analysis of existing software is needed to figure out
which use-cases are valid, and to exclude them during module scanning or =
to
provide alternative kernel interfaces to enable them. This may require a
transition phase in which module scanning would only issue warnings and
would not prevent the module from being loaded.