[PATCH v2 2/6] 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.

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()
- Thomas : Commit log changes.
---
 arch/x86/kernel/cpu/microcode/core.c | 42 +++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 0051ebf7c53e..e2cdf3e989e7 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -432,12 +432,28 @@ static int __reload_late(void *info)
 	return ret;
 }
 
+static void 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.
  */
-static void microcode_check(void)
+static void microcode_check(struct cpuinfo_x86 *orig)
 {
 	struct cpuinfo_x86 info;
 
@@ -447,15 +463,13 @@ static void microcode_check(void)
 	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));
+	* 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().
+	*/
+	store_cpu_caps(&info);
 
-	get_cpu_cap(&info);
-
-	if (!memcmp(&info.x86_capability, &boot_cpu_data.x86_capability,
+	if (!memcmp(&info.x86_capability, &orig->x86_capability,
 		    sizeof(info.x86_capability)))
 		return;
 
@@ -470,6 +484,7 @@ static void microcode_check(void)
 static int microcode_reload_late(void)
 {
 	int old = boot_cpu_data.microcode, ret;
+	struct cpuinfo_x86 info;
 
 	pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
 	pr_err("You should switch to early loading, if possible.\n");
@@ -477,9 +492,16 @@ 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.
+	 */
+	store_cpu_caps(&info);
+
 	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
 	if (ret == 0)
-		microcode_check();
+		microcode_check(&info);
 
 	pr_info("Reload completed, microcode revision: 0x%x -> 0x%x\n",
 		old, boot_cpu_data.microcode);
-- 
2.34.1