Re: __schedule #DF splat

Jan Kiszka
2014-06-29T14:33:08Z

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On 2014-06-29 16:27, Gleb Natapov wrote:
> On Sun, Jun 29, 2014 at 04:01:04PM +0200, Borislav Petkov wrote:
>> On Sun, Jun 29, 2014 at 04:42:47PM +0300, Gleb Natapov wrote:
>>> Please do so and let us know.
>>
>> Yep, just did. Reverting ae9fedc793 fixes the issue.
>>
>>> reinj:1 means that previous injection failed due to another #PF that
>>> happened during the event injection itself This may happen if GDT or fist
>>> instruction of a fault handler is not mapped by shadow pages, but here
>>> it says that the new page fault is at the same address as the previous
>>> one as if GDT is or #PF handler is mapped there. Strange. Especially
>>> since #DF is injected successfully, so GDT should be fine. May be wrong
>>> cpl makes svm crazy?
>>
>> Well, I'm not going to even pretend to know kvm to know *when* we're
>> saving VMCB state but if we're saving the wrong CPL and then doing the
>> pagetable walk, I can very well imagine if the walker gets confused. One
>> possible issue could be U/S bit (bit 2) in the PTE bits which allows
>> access to supervisor pages only when CPL < 3. I.e., CPL has effect on
>> pagetable walk and a wrong CPL level could break it.
>>
>> All a conjecture though...
>>
> Looks plausible, still strange that second #PF is at the same address as the first one though.
> Anyway, not we have the commit to blame.

I suspect there is a gap between cause and effect. I'm tracing CPL
changes currently, and my first impression is that QEMU triggers an
unwanted switch from CPL 3 to 0 on vmport access:

qemu-system-x86-11883 [001] 7493.378630: kvm_entry: vcpu 0
qemu-system-x86-11883 [001] 7493.378631: bprint: svm_vcpu_run: entry cpl 0
qemu-system-x86-11883 [001] 7493.378636: bprint: svm_vcpu_run: exit cpl 3
qemu-system-x86-11883 [001] 7493.378637: kvm_exit: reason io rip 0x400854 info 56580241 400855
qemu-system-x86-11883 [001] 7493.378640: kvm_emulate_insn: 0:400854:ed (prot64)
qemu-system-x86-11883 [001] 7493.378642: kvm_userspace_exit: reason KVM_EXIT_IO (2)
qemu-system-x86-11883 [001] 7493.378655: bprint: kvm_arch_vcpu_ioctl_get_sregs: ss.dpl 0
qemu-system-x86-11883 [001] 7493.378684: bprint: kvm_arch_vcpu_ioctl_set_sregs: ss.dpl 0
qemu-system-x86-11883 [001] 7493.378685: bprint: svm_set_segment: cpl = 0
qemu-system-x86-11883 [001] 7493.378711: kvm_pio: pio_read at 0x5658 size 4 count 1 val 0x3442554a

Yeah... do we have to manually sync save.cpl into ss.dpl on get_sregs
on AMD?

Jan

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