[PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

[PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

[Wanpeng Li]

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>
Reviewed-by: David Hildenbrand <david@redhat.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>
---
v2 -> v3:
 * move Oring X86_EFLAGS_FIXED to kvm_arch_vcpu_ioctl_set_regs
v1 -> v2:
 * Oring X86_EFLAGS_FIXED

 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ebe0a3a..0c5d55c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7405,7 +7405,7 @@ int kvm_arch_vcpu_ioctl_set_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
 #endif
 
 	kvm_rip_write(vcpu, regs->rip);
-	kvm_set_rflags(vcpu, regs->rflags);
+	kvm_set_rflags(vcpu, regs->rflags | X86_EFLAGS_FIXED);
 
 	vcpu->arch.exception.pending = false;
 
-- 
2.7.4

Re: [PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

[Quan Xu]

On 2017/12/07 16:30, 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>
> Reviewed-by: David Hildenbrand <david@redhat.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>
   Reviewed-by: Quan Xu <quan.xu0@gmail.com>