On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner tglx@linutronix.de wrote:
Andy Lutomirski luto@kernel.org writes:
On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski luto@kernel.org wrote:
/** We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current* KVM paravirt ABI, the following scenario is possible:** #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT)* NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT* NMI accesses user memory, e.g. due to perf* #PF: normal page fault* #PF reads CR2 and apf_reason -- apf_reason should be 0** outer #PF reads CR2 and apf_reason -- apf_reason should be* KVM_PV_REASON_PAGE_NOT_PRESENT** There is no possible way that both reads of CR2 and* apf_reason get the correct values. Fixing this would* require paravirt ABI changes.*/Upon re-reading my own comment, I think the problem is real, but I don't think my patch fixes it. The outer #PF could just as easily have come from user mode. We may actually need the NMI code (and perhaps MCE and maybe #DB too) to save, clear, and restore apf_reason. If we do this, then maybe CPL0 async PFs are actually okay, but the semantics are so poorly defined that I'm not very confident about that.
I think even with the current mode this is fixable on the host side when it keeps track of the state.
The host knows exactly when it injects a async PF and it can store CR2 and reason of that async PF in flight.
On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0 then it knows that the guest has read CR2 and apf_reason. All good nothing to worry about.
If not it needs to be careful.
As long as the apf_reason of the last async #PF is not cleared by the guest no new async #PF can be injected. That's already correct because in that case IF==0 which prevents a nested async #PF.
If MCE, NMI trigger a real pagefault then the #PF injection needs to clear apf_reason and set the correct CR2. When that #PF returns then the old CR2 and apf_reason need to be restored.
How is the host supposed to know when the #PF returns? Intercepting IRET sounds like a bad idea and, in any case, is not actually a reliable indication that #PF returned.