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

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Date:   Wed, 8 Apr 2020 10:58:53 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        hch@...radead.org, sean.j.christopherson@...el.com,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kenny@...ix.com, jeyu@...nel.org, rasmus.villemoes@...vas.dk,
        fenghua.yu@...el.com, xiaoyao.li@...el.com, nadav.amit@...il.com,
        thellstrom@...are.com, tony.luck@...el.com,
        gregkh@...uxfoundation.org, jannh@...gle.com,
        keescook@...omium.org, David.Laight@...lab.com,
        dcovelli@...are.com, mhiramat@...nel.org,
        Wolfgang Mauerer <wolfgang.mauerer@...-regensburg.de>
Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

On 08.04.20 10:03, Paolo Bonzini wrote:
> On 08/04/20 07:58, Jan Kiszka 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;
>>>> +        }
>>>
>>> Hmm, wont this break jailhouse?
>>
>> Yes, possibly. We load the hypervisor binary via request_firmware into
>> executable memory and then jump into it. So most of the "suspicious"
>> code is there - except two cr4_init_shadow() calls to propagate the
>> non-transparent update of VMXE into that shadow. We could hide that CR4
>> flag, but that could mislead root Linux to try to use VMX while in jail.
> 
> Why not contribute the Jailhouse loader into Linux?
> 

Definitely planned. But right now it would add the burden of managing 
the interface between loader and hypervisor carefully. Currently it is 
internal to Jailhouse and maintained in lock-step, without any backward 
compatibility.

Jan

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