We found this problem when booting a KVM guest through tboot from a
host OS where the VMX register is not exposed to the guest, even when
the guest has cloned the host CPU.
Attempting to read MSR_IA32_FEATURE_CONTROL before checking whether it
exists, on CPUs where it does not exist, sends the BSP into an infinite
loop. #GP is asserted when trying to read the non-existent MSR, which
resets the IP, only to again encounter the attempted read of the
non-existent MSR.
Postponing the read of MSR_IA32_FEATURE_CONTROL until the existence of
VMX has been ascertained prevents this problem.
Signed-off-by: Bandan Das <b...@redhat.com>
Signed-off-by: Tony Camuso <tcam...@redhat.com>
--- tboot/txt/verify.c.orig 2016-05-02 13:32:25.144003225 -0400
+++ tboot/txt/verify.c 2016-05-04 13:25:27.614166207 -0400
@@ -109,8 +109,6 @@ static bool read_processor_info(void)
}
g_cpuid_ext_feat_info = cpuid_ecx(1);
- g_feat_ctrl_msr = rdmsr(MSR_IA32_FEATURE_CONTROL);
- printk(TBOOT_DETA"IA32_FEATURE_CONTROL_MSR: %08lx\n", g_feat_ctrl_msr);
return true;
}
@@ -123,7 +121,13 @@ static bool supports_vmx(void)
}
printk(TBOOT_INFO"CPU is VMX-capable\n");
- /* and that VMX is enabled in the feature control MSR */
+ /* Now that we know we support VMX, it is safe to read the feature
+ * control MSR.
+ */
+ g_feat_ctrl_msr = rdmsr(MSR_IA32_FEATURE_CONTROL);
+ printk(TBOOT_DETA"IA32_FEATURE_CONTROL_MSR: %08lx\n", g_feat_ctrl_msr);
+
+ /* check that VMX is enabled in the feature control MSR */
if ( !(g_feat_ctrl_msr & IA32_FEATURE_CONTROL_MSR_ENABLE_VMX_IN_SMX) ) {
printk(TBOOT_ERR"ERR: VMXON disabled by feature control MSR (%lx)\n",
g_feat_ctrl_msr);
------------------------------------------------------------------------------
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