[PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Binbin Wu]

The kernel now reads MSR_IA32_PLATFORM_ID during CPU init.  When
running as a guest, if the underlying hypervisor does not emulate the
MSR, the RDMSR from MSR_IA32_PLATFORM_ID can trigger an unchecked MSR
access during early boot.  This MSR is not emulated in the case for KVM
TDX, where the following is observed in the TD guest:

    unchecked MSR access error: RDMSR from 0x17 at rIP: 0xffffffffba38d6fc (intel_get_platform_id+0x7c/0xb0)
    Call Trace:
     <TASK>
     ? early_init_intel+0x28/0x2c0
     ? early_cpu_init+0x9b/0x930
     ? setup_arch+0xbf/0xbb0
     ? _printk+0x6b/0x90
     ? start_kernel+0x7f/0xaa0
     ? x86_64_start_reservations+0x24/0x30
     ? x86_64_start_kernel+0xda/0xe0
     ? common_startup_64+0x13e/0x141
     </TASK>

The platform ID is used for one thing and one thing only: microcode
updates.  Those updates are solely the domain of the bare-metal OS.  The
guest kernel code should not even try to touch the MSR.  Skip reading
the MSR when the kernel is running in a virtualized environment.  0 is a
valid platform ID, however, microcode related logic is skipped in a
virtualized environment.

Since intel_get_platform_id() could be called early before cpuinfo_x86
is fully initialized in the case of CONFIG_MICROCODE_DBG, check whether
the kernel is running in a virtualized environment from CPUID.  Use
cpuid_ecx() instead of native_cpuid_ecx() so that Xen PV guest will see
the virtualized bit.

Fixes: d8630b67ca1ed ("x86/cpu: Add platform ID to CPU info structure")
Reported-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
v2:
- Drop the patch on KVM side. (Sean, Dave)
- Use X86_FEATURE_HYPERVISOR for better readability. (Dave)
- Use cpuid_ecx() instead of native_cpuid_ecx() to check the hypervisor bit.
- Add RB from Rick.
---
 arch/x86/kernel/cpu/microcode/intel.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 37ac4afe0972..1bc0c350726c 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -147,6 +147,10 @@ u32 intel_get_platform_id(void)
 	if (intel_cpuid_vfm() <= INTEL_PENTIUM_II_KLAMATH)
 		return 0;
 
+	/* Don't try to read microcode bits when virtualized. */
+	if (cpuid_ecx(1) & BIT(X86_FEATURE_HYPERVISOR & 0x1f))
+		return 0;
+
 	/* get processor flags from MSR 0x17 */
 	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
 

base-commit: 9974969c14031a097d6b45bcb7a06bb4aa525c40
-- 
2.46.0