[PATCH] KVM: x86: Disallow hypercalls for guest callers in rings > 0

[Jan Kiszka <jan.kiszka@siemens.com>]

So far unprivileged guest callers running in ring 3 can issue, e.g., MMU
hypercalls. Normally, such callers cannot provide any hand-crafted MMU
command structure as it has to be passed by its physical address, but
they can still crash the guest kernel by passing random addresses.

To close the hole, this patch considers hypercalls valid only if issued
from guest ring 0. This may still be relaxed on a per-hypercall base in
the future once required.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 arch/x86/kvm/x86.c       |    8 ++++++++
 include/linux/kvm_para.h |    1 +
 2 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2539e9a..966d309 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3190,6 +3190,7 @@ static inline gpa_t hc_gpa(struct kvm_vcpu *vcpu, unsigned long a0,
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 {
 	unsigned long nr, a0, a1, a2, a3, ret;
+	struct kvm_segment cs;
 	int r = 1;
 
 	nr = kvm_register_read(vcpu, VCPU_REGS_RAX);
@@ -3208,6 +3209,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 		a3 &= 0xFFFFFFFF;
 	}
 
+	kvm_get_segment(vcpu, &cs, VCPU_SREG_CS);
+	if (cs.dpl != 0) {
+		ret = -KVM_EPERM;
+		goto out;
+	}
+
 	switch (nr) {
 	case KVM_HC_VAPIC_POLL_IRQ:
 		ret = 0;
@@ -3219,6 +3226,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 		ret = -KVM_ENOSYS;
 		break;
 	}
+out:
 	kvm_register_write(vcpu, VCPU_REGS_RAX, ret);
 	++vcpu->stat.hypercalls;
 	return r;
diff --git a/include/linux/kvm_para.h b/include/linux/kvm_para.h
index 3ddce03..d731092 100644
--- a/include/linux/kvm_para.h
+++ b/include/linux/kvm_para.h
@@ -13,6 +13,7 @@
 #define KVM_ENOSYS		1000
 #define KVM_EFAULT		EFAULT
 #define KVM_E2BIG		E2BIG
+#define KVM_EPERM		EPERM
 
 #define KVM_HC_VAPIC_POLL_IRQ		1
 #define KVM_HC_MMU_OP			2