[PATCH v3 2/6] x86/microcode/core: Take a snapshot before and after applying microcode

From: Ashok Raj
Date: Tue Jan 03 2023 - 13:03:00 EST


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 | 31 +++++++++++++++++++++-------
arch/x86/kernel/cpu/microcode/core.c | 7 +++++++
3 files changed, 31 insertions(+), 8 deletions(-)

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..7c86c6fd07ae 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2297,28 +2297,43 @@ 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
* hotplug lock.
*/
-void microcode_check(struct cpuinfo_x86 *info)
+void microcode_check(struct cpuinfo_x86 *orig)
{
- perf_check_microcode();
+ struct cpuinfo_x86 info;

- /* Reload CPUID max function as it might've changed. */
- info->cpuid_level = cpuid_eax(0);
+ perf_check_microcode();

/*
* 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);
+ microcode_store_cpu_caps(&info);

- if (!memcmp(&info->x86_capability, &boot_cpu_data.x86_capability, sizeof(info->x86_capability)))
+ if (!memcmp(&info.x86_capability, &orig->x86_capability,
+ sizeof(info.x86_capability)))
return;

pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
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