From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> commit e1d38b63acd843cfdd4222bf19a26700fd5c699e upstream. Export the IA32_ARCH_CAPABILITIES MSR bit MDS_NO=0 to guests on TSX Async Abort(TAA) affected hosts that have TSX enabled and updated microcode. This is required so that the guests don't complain, "Vulnerable: Clear CPU buffers attempted, no microcode" when the host has the updated microcode to clear CPU buffers. Microcode update also adds support for MSR_IA32_TSX_CTRL which is enumerated by the ARCH_CAP_TSX_CTRL bit in IA32_ARCH_CAPABILITIES MSR. Guests can't do this check themselves when the ARCH_CAP_TSX_CTRL bit is not exported to the guests. In this case export MDS_NO=0 to the guests. When guests have CPUID.MD_CLEAR=1, they deploy MDS mitigation which also mitigates TAA. Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Neelima Krishnan <neelima.krishnan@xxxxxxxxx> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kvm/x86.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1050,6 +1050,25 @@ u64 kvm_get_arch_capabilities(void) if (!boot_cpu_has_bug(X86_BUG_MDS)) data |= ARCH_CAP_MDS_NO; + /* + * On TAA affected systems, export MDS_NO=0 when: + * - TSX is enabled on the host, i.e. X86_FEATURE_RTM=1. + * - Updated microcode is present. This is detected by + * the presence of ARCH_CAP_TSX_CTRL_MSR and ensures + * that VERW clears CPU buffers. + * + * When MDS_NO=0 is exported, guests deploy clear CPU buffer + * mitigation and don't complain: + * + * "Vulnerable: Clear CPU buffers attempted, no microcode" + * + * If TSX is disabled on the system, guests are also mitigated against + * TAA and clear CPU buffer mitigation is not required for guests. + */ + if (boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM) && + (data & ARCH_CAP_TSX_CTRL_MSR)) + data &= ~ARCH_CAP_MDS_NO; + return data; }
