Re: [PATCH v2] KVM: X86: Fix load RFLAGS w/o the fixed bit

[Wanpeng Li]

2017-12-07 1:14 GMT+08:00 Jim Mattson <jmattson@xxxxxxxxxx>:
> What if I change the testcase so that regs.rflags is initialized to (1 << 3)?

>From Paolo's pointing:

> I suspect somebody might be passing an all-zero regs struct to
> KVM_SET_REGS, so ORing X86_EFLAGS_FIXED is better.

I think the patch tries to fix a case when the userspace forgets to
set the fixed bit of rflags instead of set invalidate guest state
deliberately.

Regards,
Wanpeng Li

>
> On Wed, Dec 6, 2017 at 6:40 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
>> On 06.12.2017 12:59, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>>  *** Guest State ***
>>>  CR0: actual=0x0000000000000030, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
>>>  CR4: actual=0x0000000000002050, shadow=0x0000000000000000, gh_mask=ffffffffffffe871
>>>  CR3 = 0x00000000fffbc000
>>>  RSP = 0x0000000000000000  RIP = 0x0000000000000000
>>>  RFLAGS=0x00000000         DR7 = 0x0000000000000400
>>>         ^^^^^^^^^^
>>>
>>> The failed vmentry is triggered by the following testcase when ept=Y:
>>>
>>>     #include <unistd.h>
>>>     #include <sys/syscall.h>
>>>     #include <string.h>
>>>     #include <stdint.h>
>>>     #include <linux/kvm.h>
>>>     #include <fcntl.h>
>>>     #include <sys/ioctl.h>
>>>
>>>     long r[5];
>>>     int main()
>>>     {
>>>       r[2] = open("/dev/kvm", O_RDONLY);
>>>       r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
>>>       r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
>>>       struct kvm_regs regs = {
>>>               .rflags = 0,
>>>       };
>>>       ioctl(r[4], KVM_SET_REGS, &regs);
>>>       ioctl(r[4], KVM_RUN, 0);
>>>     }
>>>
>>> X86 RFLAGS bit 1 is fixed set, userspace can simply clearing bit 1
>>> of RFLAGS with KVM_SET_REGS ioctl which results in vmentry fails.
>>> This patch fixes it by oring X86_EFLAGS_FIXED during ioctl.
>>>
>>> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>> ---
>>> v1 -> v2:
>>>  * Oring X86_EFLAGS_FIXED
>>>
>>>  virt/kvm/kvm_main.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index b55bad3..0f3f283 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -2602,6 +2602,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
>>>                       r = PTR_ERR(kvm_regs);
>>>                       goto out;
>>>               }
>>> +             kvm_regs->rflags |= X86_EFLAGS_FIXED;
>>>               r = kvm_arch_vcpu_ioctl_set_regs(vcpu, kvm_regs);
>>>               kfree(kvm_regs);
>>>               break;
>>>
>>
>> Not sure if failing KVM_SET_REGS would be nicer, but maybe this has
>> already been discussed. So this should be fine.
>>
>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>>
>> --
>>
>> Thanks,
>>
>> David / dhildenb