linux-kernel - Re: [PATCH] deal with interrupt shadow state for emulated instruction

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Message-ID: <49E6F7D6.8010107@redhat.com>
Date:	Thu, 16 Apr 2009 12:18:14 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] deal with interrupt shadow state for emulated instruction

H. Peter Anvin wrote:
> Avi Kivity wrote:
>   
>> Why do we care?  The guest can only harm itself, and if it wants to
>> disable interrupts, it would be a lot easier for it to run a plain 'cli'.
>>
>> I guess it would be a problem if we emulated 'mov ss' for ordinary
>> userspace or vm86 mode, but we don't.
>>
>>     
>
> Well, the answer is that mov ss is an unprivileged instruction.
>
>   

We don't emulate guest user mode.

Well, if guest userspace can convince its kernel to give it access to 
some memory mapped I/O register, I guess it can execute repeated 'mov 
ss, mmio' and starve the guest kernel.

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

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