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

[Jim Mattson]

What if I change the testcase so that regs.rflags is initialized to (1 << 3)?

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