Re: [PATCH] KVM: x86: Fix TSS size check for 16-bit tasks

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> On 03/22/2010 12:29 PM, Jan Kiszka wrote:
>> A 16-bit TSS is only 44 bytes long. So make sure to test for the correct
>> size on task switch.
>>    
> 
>> This should be stable material as well. I can provide a patch that
>> applies on .32 and .33, or what will be the procedure?
>>    
> 
> I'd like to drop the Cc: stable and maintain stable queues explicitly
> (in kvm-updates/2.6.3[23]).  I'll fast-forward these to current -stable,
> please send patches against them.  These branches will be autotested
> before submission, a step that is missing in the current scheme of things.
> 
>>
>> -    if (!next_tss_desc.p || desc_limit_scaled(&next_tss_desc)<  0x67) {
>> +    desc_limit = desc_limit_scaled(&next_tss_desc);
>> +    if (!next_tss_desc.p ||
>> +        ((desc_limit<  0x67&&  (next_tss_desc.type&  8)) ||
>> +         desc_limit<  0x2c)) {
>>    
> 
> A 44-byte TSS has a limit of 43 (just like a 4GB segment has a limit of
> 0xffffffff), so there is an off-by-one here.
> 

Right - you just found an (harmless) off-by-one in our legacy OS as well
(I blindly copied its limit).

Jan


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