It was discovered that microprocessors utilizing speculative execution and branch prediction may allow unauthorized memory reads via sidechannel attacks. This flaw is known as Spectre. An attacker in the guest could use this to expose sensitive guest information, including kernel memory.
This update allows QEMU to expose new CPU features added by microcode updates to guests on amd64, i386, and s390x. On amd64 and i386, new CPU models that match the updated microcode features were added with an -IBRS suffix. Certain environments will require guests to be switched manually to the new CPU models after microcode updates have been applied to the host.
2.0.0+dfsg-2ubuntu1.381:2.5+dfsg-5ubuntu10.20