[PATCH 4.4 11/63] KVM: X86: Fix load RFLAGS w/o the fixed bit

Greg Kroah-Hartman
2018-01-01T14:24:29Z

From: Wanpeng Li <wanpeng.li@hotmail.com>

commit d73235d17ba63b53dc0e1051dbc10a1f1be91b71 upstream.

*** 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>
Reviewed-by: Quan Xu <quan.xu0@gmail.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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6941,7 +6941,7 @@ int kvm_arch_vcpu_ioctl_set_regs(struct
#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;