Re: [PATCH 1/3] KVM: x86 emulator: Fix task switch privilege checks

[Kevin Wolf <kwolf@redhat.com>]

Am 24.01.2012 15:16, schrieb Gleb Natapov:
> On Tue, Jan 24, 2012 at 03:15:13PM +0100, Kevin Wolf wrote:
>> Am 24.01.2012 15:03, schrieb Joerg Roedel:
>>> On Mon, Jan 23, 2012 at 05:10:46PM +0100, Kevin Wolf wrote:
>>>> This patch fixes the problem for VMX. For SVM, the logic used to
>>>> determine the source of the task switch is buggy, so we can't pass
>>>> useful information to the emulator there and just disable the check in
>>>> all cases.
>>>
>>> Actually, SVM isn't buggy :) For SVM you do not need to do any
>>> priviledge checks in software because the hardware already takes care of
>>> that.
>>> In other words, KVM only gets a task-switch intercept if the priviledges
>>> are all checked and correct.
>>
>> Okay, that's good to hear. The current code is still buggy because as
>> Gleb noted it checks against the TSS DPL. We need to disable that check
>> for SVM then. Also all checks for TASK_SWITCH_GATE indicate that
>> something is wrong because it will never happen.
>>
> Not necessary. Currently all checks for TASK_SWITCH_GATE also check for
> TASK_SWITCH_CALL, so I think you can fix SVM case in your patch by
> passing TASK_SWITCH_GATE instead of TASK_SWITCH_CALL to
> kvm_task_switch().

Yes, the emulator itself would be fixed by passing TASK_SWITCH_GATE and
idt_index = -1 (although it looks a bit brittle).

However, task_switch_interception() itself does some more based on the
value of reason, for example it decides whether or not to call
skip_emulated_instruction().

Kevin