Re: KVM: Fix the invlpg instruction emulation on AMD64

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Aurelien Jarno wrote:
> On Tue, Oct 16, 2007 at 12:12:18PM +0200, Avi Kivity wrote:
>   
>> Aurelien Jarno wrote:
>>     
>>> I have marked the invlpg instruction the same way as it is done in
>>> kvm-37 to know what happens. I get either modrm_reg = 4 or = 6 when the
>>> invlpg instruction is executed, but never = 7.
>>>
>>>  
>>>       
>> Then it isn't the invlpg instruction at all.  Rather smsw (modrm_reg == 
>> 4) or lmsw ( == 6).
>>
>> I'm confused.
>>
>> (looks)
>>
>> Okay.  What we have here is total breakage when emulating an instruction 
>> that uses a mod r/m encoding that actually refers to a register 
>> (modrm_mod == 3).  In x86_decode_insn() we set src.type as OP_MEM, and 
>> in x86_emulate_insn() we happily fetch it even though it's a register, 
>> generating a fault.
>>
>> It usually doesn't bite us because these instructions are directly executed.
>>
>> The fix is probably to switch to OP_REG if SrcMem and ModRM and 
>> modrm_mod == 3 (similarly for DstMem).
>>
>>     
>
> Ok, I have tried to implement that for SrcMem, and it works in my case.
> I am able to boot FreeBSD, and I am seeing no regression for Linux or
> Hurd VMs. Does the patch below looks ok to you?
>
>
>
> KVM: Fix the invlpg instruction emulation on AMD64
>
> The patch below correctly detects the invlpg instruction, and switch 
> src.type to OP_REG. This fixes the boot of FreeBSD on an AMD64 CPU, it has
> been broken since commit aa38840d3d2e0a804e628077df8d8879b496d741.
>
> It also moves the assignation of c->src.bytes after the test as it is
> not needed for the invlpg instruction.
>
> Signed-off-by: Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>
>
> diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
> index e974ace..06e183b 100644
> --- a/drivers/kvm/x86_emulate.c
> +++ b/drivers/kvm/x86_emulate.c
> @@ -825,12 +825,12 @@ modrm_done:
>  		c->src.bytes = 4;
>  		goto srcmem_common;
>  	case SrcMem:
> -		c->src.bytes = (c->d & ByteOp) ? 1 :
> -							   c->op_bytes;
>  		/* Don't fetch the address for invlpg: it could be unmapped. */
> -		if (c->twobyte && c->b == 0x01
> -				    && c->modrm_reg == 7)
> +		if ((c->d & ModRM) && c->modrm_mod == 3) {
> +			c->src.type = OP_REG;
>  			break;
> +		}
> +		c->src.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
>  srcmem_common:
>  		c->src.type = OP_MEM;
>  		break;
>   

Why did you remove the check for invlpg?  It need not be executed with
modrm_mod == 3 (in fact invlpg with modrm_mod == 3 is useless), so it
won't be caught by this.

Also, we need to initialize the fields of src in this case.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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