Andy Lutomirski <luto@xxxxxxxxxx> writes: > On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski <luto@xxxxxxxxxx> 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. I tried to figure out whether any of this logic exists in the KVM code, but I got completely lost in that code. Maybe I try later today again. Thanks, tglx
