linux-kernel - Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

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Message-Id: <F4B5EE1A-AD3C-4098-8784-E19356A25A48@gmail.com>
Date:   Tue, 7 Apr 2020 16:51:54 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, hch@...radead.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        mingo <mingo@...hat.com>, bp <bp@...en8.de>, hpa@...or.com,
        x86 <x86@...nel.org>, "Kenneth R. Crudup" <kenny@...ix.com>,
        Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jannh@...gle.com, keescook@...omium.org, David.Laight@...lab.com,
        Doug Covelli <dcovelli@...are.com>, mhiramat@...nel.org
Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

> On Apr 7, 2020, at 3:12 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 07/04/20 23:27, Peter Zijlstra wrote:
>> On Tue, Apr 07, 2020 at 02:22:11PM -0700, Nadav Amit wrote:
>>> 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.
>> That needs an exported function; there is no way we'll allow random
>> writes to CR4, there's too much dodgy stuff in there.
> 
> native_write_cr4 and pv_ops (through which you can do write_cr4) are
> both exported, and so is cpu_tlbstate which is used by __cr4_set_bits
> and friends.  Am I missing something glaringly obvious?

No, I was the one who missed the obvious thing.

Having said that, I still think there are additional cases that need to be
handled. For instance, I see that is_erratum_383() in KVM (AMD) flushes the
local TLB by writing to CR3 the previous value. I am not familiar with the
erratum. Maybe I am missing something again, but I do not see an appropriate
exported alternative in the kernel.