2017-10-26 14:52 GMT+08:00 Nadav Amit <nadav.amit@xxxxxxxxx>: > Wanpeng Li <kernellwp@xxxxxxxxx> wrote: > >> 2017-10-26 0:20 GMT+08:00 Nadav Amit <nadav.amit@xxxxxxxxx>: >>> Wanpeng Li <kernellwp@xxxxxxxxx> wrote: >>> >>>> Cc Radim, Nadav, >>>> 2017-10-24 19:10 GMT+08:00 Pedro Fonseca <pfonseca@xxxxxxxxxxxxxxxxx>: >>>>> Hi, >>>>> >>>>> During tests that we conducted on KVM, we noticed that executing a "PUSH >>>>> %ES" instruction under KVM produces different results on both memory and the >>>>> SP register depending on whether EPT support is enabled. With EPT the SP is >>>>> reduced by 4 bytes (and the written value is 0-padded) but without EPT >>>>> support it is only reduced by 2 bytes. The difference can be observed when >>>>> the CS.DB field is 1 (32-bit) but not when it's 0 (16-bit). >>>>> >>>>> The test case initializes the VM with EIP=0, CS.DB=1, ES=0x10, and SP=0xFFE. >>>>> Memory is initialized with 0x06 (PUSH %ES) and 0xF4 (HLT). The testing >>>>> system was running Linux 4.12.5 and Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz. >>>>> >>>>> The test case (https://pastebin.com/ZejdtGEk) produces the output bellow. >>>>> Note that 0x10 is written to 0xFFA on EPT=1 but it's written to 0xFFC on >>>>> EPT=0. >>>>>> $ insmod kvm-intel.ko >>>>>> $ sudo ./reproduce-push_es >>>>>> Executing KVM_RUN >>>>>> KVM_RUN exited (exit_reason: 5, KVM_EXIT_HLT) >>>>>> 0000: 06 f4 00 00 00 00 00 00 >>>>>> 0008: 00 00 00 00 00 00 00 00 >>>>>> 0ff8: 00 00 10 00 00 00 00 00 >>>>>> 1000: 00 00 00 00 00 00 00 00 >>>>> >>>>> >>>>>> $ insmod kvm-intel.ko ept=0 >>>>>> $ sudo ./reproduce-push_es >>>>>> Executing KVM_RUN >>>>>> KVM_RUN exited (exit_reason: 5, KVM_EXIT_HLT) >>>>>> 0000: 06 f4 00 00 00 00 00 00 >>>>>> 0008: 00 00 00 00 00 00 00 00 >>>>>> 0ff8: 00 00 00 00 10 00 00 00 >>>>>> 1000: 00 00 00 00 00 00 00 00 >>>> >>>> The cause of your two reports are the same. I think it has associated >>>> with EPT+unrestricted_guest and vm8086 instead of EPT itself. vm8086 >>>> emulates a real mode environment, so it will not respect CS.D=1 which >>>> you give since there is no segment descriptors support. However, big >>>> real mode is different, they still load the segment descriptors which >>>> hand over from protect mode before the mode switch. Your testcase just >>>> start a real mode guest in all its life time w/o switch to protect >>>> mode or vice versa. And KVM(EPT=Y, unrestricted_guest=Y) can't >>>> distinguish between a real mode guest w/ segment descriptors given by >>>> userspace and big real mode which occurs when protect mode switch to >>>> real mode. >>> >>> Interesting. I can guess that the Intel tests that I was running back at the >>> time had a setup code (prior to the random code) in protected-mode, which >>> would explain why I missed this problem. >>> >>> Perhaps the problem comes from wrong setting of the guest segment selector >>> “unusable” bit. I see there are quite few hacks in the code in regard to >>> this bit. >> >> I change the "present" bit of CS/DS/SS/ES to 0 in the testcase, >> however, the guest vmentry fails. In addition, is there any idea how >> to fix it in kvm? I can be the volunteer to implement the idea. :) > > Stupid me. I didn’t read the setup well enough. So I understand there is > actually emulation when EPT=0, and this emulation is wrong. > > I don’t see where the operand size (op_bytes) for “Stack” operations in > x86_decode_insn() is updated in respect to cs.d, and there is also no > appropriate logic in em_push_sreg(). Do you mean vm8086 should still respect cs.d even if there is no segment descriptors in real mode? Regards, Wanpeng Li
