Re: [Xen-devel] XSA-60 solutions

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Liu, Jinsong wrote:
> Hi, All
> 
> This email provides 2 solutions for XSA-60 issue found by Konrad
> (refer attached email for XSA-60 please). 
> 
> Basically it involves how to emulate guest setting cr0.cd. For
> shadow, as Jan pointed out in earlier email Xen drop all shadows so
> that any new ones would be created with UC memory type, _not_
> involving iteration over the whole address space. For EPT, currently
> Xen traverse all ept entries via problematic set_uc_mode, resulting
> in DOS-like behavior, so this email focus on Intel EPT case.     
> 
> Solution 1 is Dual-EPT tables: When guest setting cr0.cd trapped,
> stop using normal EPT, switch to a temp EPT table and populate new
> EPT entries w/ UC type on demand at later EPT violation. When guest
> clearing cr0.cd, switch back to normal EPT. In this way, _no_
> unbounded loop involved and hence security hole avoided.    
> 
> Some concerns for Dual-EPT: the 1st concern comes from cachablity
> confliction between guest and Xen memory type point of view, though
> it also exists in current implementation. The 2nd concern comes from
> Dual EPT tables inconsistency/sync issue: things become complicated
> when p2m modifying, PoD populating, and super page spliting, etc.    
> 
> Solution 2 is via PAT emulation: For guest w/o VT-d, and for guest
> with VT-d but snooped, Xen need do nothing, just simply ignore guest
> setting cr0.cd, since hardware snoop mechanism has ensured cache
> coherency (under these cases currently Xen has set EPT iPAT bit,
> ignore guest's memory type opinion); For guest with VT-d but
> non-snooped, cache coherency can not be guaranteed by h/w snoop so
> guest's memory type opinion must be considered (under this case Xen
> set iPAT bit combining guest and host memory type opinion). Only

Sorry, under this case Xen _clear_ iPAT, combining guest and host memory type 
opinion.

Thanks,
Jinsong

> under this case PAT emulation need set all IA32_PAT fields as UC so
> that guest memory type are all UC.         
> 
> Concern for PAT solution still comes from cachablity confliction
> between guest and Xen. 
> 
> Thoughts?
> BTW, today is Chinese National day, I will have several days travel
> with no email access, but your comments/suggestions are highly
> appreciated and I will reply ASAP after I come back.  
> 
> Thanks,
> Jinsong


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