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SubjectRe: [PATCH v2] KVM: X86: Fix load RFLAGS w/o the fixed bit
2017-12-07 1:28 GMT+08:00 David Hildenbrand <david@redhat.com>:
> On 06.12.2017 15:40, David Hildenbrand wrote:
>> On 06.12.2017 12:59, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>>
>>> *** 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@google.com>
>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>> Cc: Jim Mattson <jmattson@google.com>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>>> ---
>>> 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;
>
> Just realized that this should go into kvm_arch_vcpu_ioctl_set_regs().

Do it in v3, thanks for your review. :)

Regards,
Wanpeng Li

>
>
>>> 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@redhat.com>
>>
> --
>
> Thanks,
>
> David / dhildenb

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