[Xen-devel] Spectre Mitigations in Xen 4.6

[Jason Andryuk <jandryuk@xxxxxxxxx>]

Hi,

I've been experimenting with Linux 4.14 on Xen 4.6.  Now that Intel
microcode is generally
available, I'm starting to exercise the new mitigation code paths.

For Xen 4.6-4.8, microcode loading happens after
init_speculation_mitigations, so Xen only
detects the boot firmware features.  The early microcode loading
f97838bbd980 ("x86: Move
microcode loading earlier") can be cherry-picked, though small fix ups
are needed for
bool/true/false -> bool_t/1/0 and smpboot.c:smp_store_cpu_info() to
retain "struct
cpuinfo_x86 *c = cpu_data + id;".

With that in place, I'm seeing Dom0 receive a general protection fault on boot

[   25.460035] general protection fault: 0000 [#1] SMP
[   25.460292] EIP: switch_mm_irqs_off+0xbe/0x600

switch_mm_irqs_off+0xbe is the inlined
indirect_branch_prediction_barrier(void)
{
        alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,
                              X86_FEATURE_USE_IBPB);
}

The system boots when dom0 disables IBPB manipulation with
nospectre_v2 on the kernel
command line.

I think Xen ends up here in xen/arch/x86/traps.c:emulate_privileged_op(),
        case MSR_PRED_CMD:
            domain_cpuid(currd, 7, 0, &dummy, &dummy, &dummy, &edx);
            domain_cpuid(currd, 0x80000008, 0, &dummy, &ebx, &dummy, &dummy);
            if ( !(edx & cpufeat_mask(X86_FEATURE_IBRSB)) &&
                 !(ebx & cpufeat_mask(X86_FEATURE_IBPB)) )
                goto fail; /* MSR available? */

            /*
             * The only defined behaviour is when writing PRED_CMD_IBPB.  In
             * practice, real hardware accepts any value without faulting.
             */
            if ( eax & PRED_CMD_IBPB )
                wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
            break;

...but Dom0 doesn't have a cpuid policy configured, so the IBRSB/IBPB
check fails and we GP.
Did I read that correctly?  If that is the case, how should Dom0 be handled?

Other 4.14 PV & HVM DomUs boot fine and detect (and use?) IBPB once
Dom0 boots (with
spectre mitigations disabled).

Regards,
Jason

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