[PATCH 2/2] KVM: X86: Fix load RFLAGS w/o the fixed bit

[Wanpeng Li]

From: Wanpeng Li <wanpeng...@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 catching userspace set RFLAGS w/o the fixes 
bit and bailing out immediately.

Suggested-by: Jim Mattson <jmatt...@google.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Radim Krčmář <rkrc...@redhat.com>
Cc: Jim Mattson <jmatt...@google.com>
Signed-off-by: Wanpeng Li <wanpeng...@hotmail.com>
---
 virt/kvm/kvm_main.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c01cff0..7100833 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2602,6 +2602,11 @@ static long kvm_vcpu_ioctl(struct file *filp,
                        r = PTR_ERR(kvm_regs);
                        goto out;
                }
+               r = -EINVAL;
+               if (!(kvm_regs->rflags & X86_EFLAGS_FIXED)) {
+                       kfree(kvm_regs);
+                       goto out;
+               }
                r = kvm_arch_vcpu_ioctl_set_regs(vcpu, kvm_regs);
                kfree(kvm_regs);
                break;
-- 
2.7.4