Re: [PATCH 0/5] KVM: x86: support setting the CPL independent of CS

[Paolo Bonzini]

Il 13/05/2014 16:55, Paolo Bonzini ha scritto:
> Until now, KVM used to assume that CS.RPL could always be used as the CPL
> value when KVM_SET_SREGS is called.  Unfortunately this is not the case.
> If userspace decides to call KVM_GET_SREGS/KVM_SET_SREGS exactly after
> CR0.PE has been set to 1, but before the long jump that reloads CS, the
> CPL will be reset to bits 0-1 of CS (aka CS.RPL).  This can work or not,
> depending on the placement of the code that transitions to protected
> mode.  If CS.RPL != 0 the emulator will see CS.RPL != CS.DPL (the DPL
> will always be zero) and fail to fetch the next instruction of the
> transition code.
>
> The same bug exists with SVM, where you don't have the emulator but the
> guest will triple fault.  Strangely, it doesn't occur with Intel's
> unrestricted guest mode.
>
> To trigger this using QEMU, it is enough to send "info cpus" continuously
> while running iPXE (which places its code for real->protected mode in
> the EBDA).  iPXE does a lot of transitions, and the guest will crash
> very quickly.
>
> Avi or Gleb, this is a bit tricky.  Can you review it please?

There's a simpler way to fix this, by using SS.DPL as the CPL.  Patch on 
its way...

Paolo

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