Re: [PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD

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

Yang, Sheng wrote:
> From 9743b5299bae1779c2b893cbeb86122bcccb9b2d Mon Sep 17 00:00:00 2001
> From: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> Date: Wed, 2 Jan 2008 14:49:22 +0800
> Subject: [PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD
>
> When executing a test program called "crashme", we found the KVM guest cannot 
> survived more than ten seconds, then encounterd kernel panic. The basic 
> concept of "crashme" is graduating random assembly code and trying to execute 
> them in a fork process.
>
> After some fix on emulator valid judgment, we found it's hard to get the 
> current emulator handle the invalid instructions correctly, for the #UD trap 
> for hypercall patching caused troubles. The problem is, if the opcode itself 
> was OK, but combination of opcode and modrm_reg was invalid, and one operand 
> of the opcode was memory(SrcMem or DstMem), emulator would fetched the memory 
> operand first rather than judged the validity, and may encounter error there. 
> For example, ".byte 0xfe, 0x34, 0xcd" got this trouble.
>
> In the patch, we simply check that if the invalid opcode isn't vmcall/vmmcall, 
> then return from emulate_instruction() and inject a #UD to guest. With the 
> patch, the guest had been run for more than 12 hours.
>
>   

Applied, thanks.  As Anthony says, good catch indeed.

I have a vague plan for improving decode; basically extend the decode 
tables to add group decoding.  We add a bit to opcode_table and 
twobyte_table that is set for all instructions which need group 
decoding.  When the bit is set, the rest of the value in opcode_table is 
interpreted as an index (together with modrm_reg) into a new 
group_table, so we can have different decoding for such instructions.


-- 
error compiling committee.c: too many arguments to function


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