Re: [Xen-devel] [PATCH] x86/VMX: sanitize VM86 TSS handling

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 06:19 -0700 on 13 Feb (1486966797), Jan Beulich wrote:
> The present way of setting this up is flawed: Leaving the I/O bitmap
> pointer at zero means that the interrupt redirection bitmap lives
> outside (ahead of) the allocated space of the TSS. Similarly setting a
> TSS limit of 255 when only 128 bytes get allocated means that 128 extra
> bytes may be accessed by the CPU during I/O port access processing.
> 
> Introduce a new HVM param to set the allocated size of the TSS, and
> have the hypervisor actually take care of setting namely the I/O bitmap
> pointer. Both this and the segment limit now take the allocated size
> into account.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

This looks pretty good to me.  Once the question below is resolved,
Reviewed-by: Tim Deegan <tim@xxxxxxx>

> ---
> TBD: Do we really want to re-init the TSS every time we are about to
>      use it?

No - I think we should init it when the guest writes the param(s) and
leave it at that.  Hvmloader marks it as reserved so the guest should
know better than to write to it, and we can't protect it against all
the possible ways the guest could break itself.

If you do want to re-init it more often, then I think it would still
be better to legacy guests' (lack of a) size limit once, when the guest
writes the base param.

Cheers,

Tim.

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