linux-kernel - Re: [PATCH 0/3] arm64:msr: Add MSR driver

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Date:   Tue, 01 Dec 2020 15:37:26 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     wangrongwei <rongwei.wang@...ux.alibaba.com>
Cc:     catalin.marinas@....com, Will Deacon <will@...nel.org>,
        bjorn.andersson@...aro.org, shawnguo@...nel.org, gshan@...hat.com,
        geert+renesas@...der.be, Anson.Huang@....com, masahiroy@...nel.org,
        michael@...le.cc, krzk@...nel.org, linux-kernel@...r.kernel.org,
        vkoul@...nel.org, olof@...om.net, vincenzo.frascino@....com,
        ardb@...nel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/3] arm64:msr: Add MSR driver

On 2020-12-01 14:25, wangrongwei wrote:
>> 2020年12月1日 下午4:12，Marc Zyngier <maz@...nel.org> 写道：
>> 
>> On 2020-12-01 03:09, wangrongwei wrote:
>>> Hi
>>> We have validate this driver in vm and physical machine, and works 
>>> fine.
>> 
>> But what does "work fine" mean? None of these system registers are 
>> supposed
>> to be accessible from userspace, so please explain *what* you are 
>> trying to
>> do with this, other that introducing security holes and general system
>> instability?
> I think I know what you mean. Do you want me to describe how we 
> achieved it?
> In x86, the different registers can be accessed directly using the
> rdmsr and wrmsr instructions, but in ARM, since these two instructions
> are missing, so we modify the code segment during runtime, similar to
> the principle of static_key.

[...]

These are implementation details, none of which answer my question:

What makes you think this is a good idea? I cannot see any legitimate
reason for userspace to ever access privileged system registers, and
the fact that x86 has such feature isn't a good justification.

         M.
-- 
Jazz is not dead. It just smells funny...