Re: [Xen-devel] [PATCH] x86/HVM: correct notion of new CPL in task switch emulation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/06/17 13:11, Jan Beulich wrote:
> Commit aac1df3d03 ("x86/HVM: introduce hvm_get_cpl() and respective
> hook") went too far in one aspect: When emulating a task switch we
> really shouldn't be looking at what hvm_get_cpl() returns, as we're
> switching all segment registers.
>
> However, instead of reverting the relevant parts of that commit, have
> the caller tell the segment loading function what the new CPL is. This
> at once fixes ES being loaded before CS so far having had its checks
> done against the old CPL.
>
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

On further consideration, wouldn't it be better to audit all segment
registers, before updating any of them in the vmcs/vmcb?  This would
leave us with a far lower chance of other vmentry failures.

Loading the segment registers is beyond the commit point of a task
switch, and the manual says that the processor will try to skip further
segmentation checks in an attempt to deliver a fault in the new context.

~Andrew

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