[PATCH v3 2/2] x86/microcode/core: Take a snapshot before and after applying microcode

[Ashok Raj]

The kernel caches features about each CPU's features at boot in an
x86_capability[] structure. The microcode update takes one snapshot and
compares it with the saved copy at boot.

However, the capabilities in the boot copy can be turned off as a result of
certain command line parameters or configuration restrictions. This can
cause a mismatch when comparing the values before and after the microcode
update.

microcode_check() is called after an update to report any previously
cached CPUID bits might have changed due to the update.

microcode_store_cpu_caps() basically stores the original CPU reported
values and not the OS modified values. This will avoid giving a false
warning even if no capabilities have changed.

Ignore the capabilities recorded at boot. Take a new snapshot before the
update and compare with a snapshot after the update to eliminate the false
warning.

Fixes: 1008c52c09dc ("x86/CPU: Add a microcode loader callback")
Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Cc: x86 <x86@xxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Alison Schofield <alison.schofield@xxxxxxxxx>
Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
Changes since last post

- Boris
	- Change function from copy_cpu_caps() -> store_cpu_caps()
	- Keep microcode_check() inside cpu/common.c and not bleed
	  get_cpu_caps() outside of core code.

- Thomas : Commit log changes.
---
 arch/x86/include/asm/processor.h     |  1 +
 arch/x86/kernel/cpu/common.c         | 17 +++++++++++++++++
 arch/x86/kernel/cpu/microcode/core.c |  7 +++++++
 3 files changed, 25 insertions(+)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 387578049de0..ac2e67156b9b 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -697,6 +697,7 @@ bool xen_set_default_idle(void);
 #endif
 
 void __noreturn stop_this_cpu(void *dummy);
+void microcode_store_cpu_caps(struct cpuinfo_x86 *info);
 void microcode_check(struct cpuinfo_x86 *info);
 
 enum l1tf_mitigations {
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index b9c7529c920e..1f0b57a5d89d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2297,6 +2297,23 @@ void cpu_init_secondary(void)
 #endif
 
 #ifdef CONFIG_MICROCODE_LATE_LOADING
+
+void microcode_store_cpu_caps(struct cpuinfo_x86 *info)
+{
+	/* Reload CPUID max function as it might've changed. */
+	info->cpuid_level = cpuid_eax(0);
+
+	/*
+	 * Copy all capability leafs to pick up the synthetic ones so that
+	 * memcmp() below doesn't fail on that. The ones coming from CPUID will
+	 * get overwritten in get_cpu_cap().
+	 */
+	memcpy(info->x86_capability, &boot_cpu_data.x86_capability,
+	       sizeof(info->x86_capability));
+
+	get_cpu_cap(info);
+}
+
 /*
  * The microcode loader calls this upon late microcode load to recheck features,
  * only when microcode has been updated. Caller holds microcode_mutex and CPU
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index d86a4f910a6b..14d9031ed68a 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -447,6 +447,13 @@ static int microcode_reload_late(void)
 	atomic_set(&late_cpus_in,  0);
 	atomic_set(&late_cpus_out, 0);
 
+	/*
+	 * Take a snapshot before the microcode update, so we can compare
+	 * them after the update is successful to check for any bits
+	 * changed.
+	 */
+	microcode_store_cpu_caps(&info);
+
 	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
 	if (ret == 0)
 		microcode_check(&info);
-- 
2.34.1