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

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.

Cc: stable@kernel.org
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fa525d511d92..92b5eddaa877 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3213,6 +3213,11 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
                 a3 &= 0xFFFFFFFF;
         }
 
+        if (kvm_x86_ops->get_cpl(vcpu) != 0) {
+                ret = -KVM_EPERM;
+                goto out;
+        }
+
         switch (nr) {
         case KVM_HC_VAPIC_POLL_IRQ:
                 ret = 0;
@@ -3224,6 +3229,7 @@ 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;