kernel: KVM: nVMX: missing IBPB when exiting from nested guest can lead to Spectre v2 attacks

A flaw was found in the KVM's Intel nested virtualization feature nVMX. Since L1 and L2 shared branch prediction modes guest-user and guest-kernel, KVM did not protect indirect branches in L1 from steering by a malicious agent in L2. This could allow a malicious nested guest to carry out Spectre ...