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Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
To:     Christoph Hellwig <hch@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>, tglx@linutronix.de,
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        gregkh@linuxfoundation.org, jannh@google.com,
        keescook@chromium.org, David.Laight@aculab.com,
        dcovelli@vmware.com, mhiramat@kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <2b0dc69c-f7f9-985d-fc40-8b7bbd927e4f@redhat.com>
Date:   Wed, 8 Apr 2020 18:15:48 +0200
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On 08/04/20 17:46, Christoph Hellwig wrote:
> On Wed, Apr 08, 2020 at 05:44:19PM +0200, Peter Zijlstra wrote:
>> On Wed, Apr 08, 2020 at 09:27:26AM -0400, Steven Rostedt wrote:
>>> On Tue, 07 Apr 2020 13:02:40 +0200
>>> Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>>> +		if (insn_is_mov_CRn(&insn) || insn_is_mov_DRn(&insn)) {
>>>> +			pr_err("Module writes to CRn or DRn, please use the proper accessors: %s\n", mod->name);
>>>> +			return -ENOEXEC;
>>>> +		}
>>>> +
>>>
>>> Something like this should be done for all modules, not just out of tree
>>> modules.
>>
>> I'm all for it; but people were worried scanning all modules was too
>> expensive (I don't really believe it is, module loading just can't be a
>> hot-path). Also, in-tree modules are audited a lot more than out of tree
>> magic voodoo crap.
> 
> 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?

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.

However, I wouldn't mind it if VMCLEAR/VMPTRLD and the associated kdump
cleanup code were moved to core kernel code.

Paolo