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Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        hch@infradead.org, sean.j.christopherson@intel.com,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        kenny@panix.com, jeyu@kernel.org, rasmus.villemoes@prevas.dk,
        fenghua.yu@intel.com, xiaoyao.li@intel.com, nadav.amit@gmail.com,
        thellstrom@vmware.com, tony.luck@intel.com,
        gregkh@linuxfoundation.org, jannh@google.com,
        keescook@chromium.org, David.Laight@aculab.com,
        dcovelli@vmware.com, mhiramat@kernel.org,
        Wolfgang Mauerer <wolfgang.mauerer@oth-regensburg.de>
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 <137fe245-69f3-080e-5f2b-207cd218f199@siemens.com>
 <0ed2739b-6961-c476-be2d-020e855796dc@redhat.com>
From:   Jan Kiszka <jan.kiszka@siemens.com>
Message-ID: <2aed2b96-c726-1357-44bb-649ec0a809ad@siemens.com>
Date:   Wed, 8 Apr 2020 10:58:53 +0200
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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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