[patch V4 29/30] x86/microcode: Prepare for minimal revision check

From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: x86@kernel.org, Borislav Petkov <bp@alien8.de>,
	"Chang S. Bae" <chang.seok.bae@intel.com>,
	Arjan van de Ven <arjan@linux.intel.com>,
	Nikolay Borisov <nik.borisov@suse.com>
Subject: [patch V4 29/30] x86/microcode: Prepare for minimal revision check
Date: Mon,  2 Oct 2023 14:00:10 +0200 (CEST)	[thread overview]
Message-ID: <20231002115903.718904225@linutronix.de> (raw)
In-Reply-To: 20231002115506.217091296@linutronix.de

From: Thomas Gleixner <tglx@linutronix.de>

Applying microcode late can be fatal for the running kernel when the update
changes functionality which is in use already in a non-compatible way,
e.g. by removing a CPUID bit.

There is no way for admins which do not have access to the vendors deep
technical support to decide whether late loading of such a microcode is
safe or not.

Intel has added a new field to the microcode header which tells the minimal
microcode revision which is required to be active in the CPU in order to be
safe.

Provide infrastructure for handling this in the core code and a command
line switch which allows to enforce it.

If the update is considered safe the kernel is not tainted and the annoying
warning message not emitted. If it's enforced and the currently loaded
microcode revision is not safe for late loading then the load is aborted.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 Documentation/admin-guide/kernel-parameters.txt |    5 +++++
 arch/x86/Kconfig                                |   23 ++++++++++++++++++++++-
 arch/x86/kernel/cpu/microcode/amd.c             |    3 +++
 arch/x86/kernel/cpu/microcode/core.c            |   19 ++++++++++++++-----
 arch/x86/kernel/cpu/microcode/intel.c           |    3 +++
 arch/x86/kernel/cpu/microcode/internal.h        |    3 +++
 6 files changed, 50 insertions(+), 6 deletions(-)
---

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3275,6 +3275,11 @@
 
 	mga=		[HW,DRM]
 
+	microcode.force_minrev=	[X86]
+			Format: <bool>
+			Enable or disable the microcode minimal revision
+			enforcement for the runtime microcode loader.
+
 	min_addr=nn[KMG]	[KNL,BOOT,IA-64] All physical memory below this
 			physical address is ignored.
 
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1322,7 +1322,28 @@ config MICROCODE_LATE_LOADING
 	  is a tricky business and should be avoided if possible. Just the sequence
 	  of synchronizing all cores and SMT threads is one fragile dance which does
 	  not guarantee that cores might not softlock after the loading. Therefore,
-	  use this at your own risk. Late loading taints the kernel too.
+	  use this at your own risk. Late loading taints the kernel unless the
+	  microcode header indicates that it is safe for late loading via the
+	  minimal revision check. This minimal revision check can be enforced on
+	  the kernel command line with "microcode.minrev=Y".
+
+config MICROCODE_LATE_FORCE_MINREV
+	bool "Enforce late microcode loading minimal revision check"
+	default n
+	depends on MICROCODE_LATE_LOADING
+	help
+	  To prevent that users load microcode late which modifies already
+	  in use features, newer microcodes have a minimum revision field
+	  in the microcode header, which tells the kernel which minimum
+	  revision must be active in the CPU to safely load that new microcode
+	  late into the running system. If disabled the check will not
+	  be enforced but the kernel will be tainted when the minimal
+	  revision check fails.
+
+	  This minimal revision check can also be controlled via the
+	  "microcode.minrev" parameter on the kernel command line.
+
+	  If unsure say Y.
 
 config X86_MSR
 	tristate "/dev/cpu/*/msr - Model-specific register support"
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -880,6 +880,9 @@ static enum ucode_state request_microcod
 	enum ucode_state ret = UCODE_NFOUND;
 	const struct firmware *fw;
 
+	if (force_minrev)
+		return UCODE_NFOUND;
+
 	if (c->x86 >= 0x15)
 		snprintf(fw_name, sizeof(fw_name), "amd-ucode/microcode_amd_fam%.2xh.bin", c->x86);
 
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -46,6 +46,9 @@
 static struct microcode_ops	*microcode_ops;
 static bool dis_ucode_ldr = true;
 
+bool force_minrev = IS_ENABLED(CONFIG_MICROCODE_LATE_FORCE_MINREV);
+module_param(force_minrev, bool, S_IRUSR | S_IWUSR);
+
 bool initrd_gone;
 
 /*
@@ -559,15 +562,17 @@ static int ucode_load_cpus_stopped(void
 	return 0;
 }
 
-static int load_late_stop_cpus(void)
+static int load_late_stop_cpus(bool is_safe)
 {
 	unsigned int cpu, updated = 0, failed = 0, timedout = 0, siblings = 0;
 	unsigned int nr_offl, offline = 0;
 	int old_rev = boot_cpu_data.microcode;
 	struct cpuinfo_x86 prev_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");
+	if (!is_safe) {
+		pr_err("Late microcode loading without minimal revision check.\n");
+		pr_err("You should switch to early loading, if possible.\n");
+	}
 
 	atomic_set(&late_cpus_in, num_online_cpus());
 	atomic_set(&offline_in_nmi, 0);
@@ -617,7 +622,9 @@ static int load_late_stop_cpus(void)
 		return -EIO;
 	}
 
-	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+	if (!is_safe || failed || timedout)
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
 	pr_info("load: updated on %u primary CPUs with %u siblings\n", updated, siblings);
 	if (failed || timedout) {
 		pr_err("load incomplete. %u CPUs timed out or failed\n",
@@ -707,7 +714,9 @@ static int load_late_locked(void)
 
 	switch (microcode_ops->request_microcode_fw(0, &microcode_pdev->dev)) {
 	case UCODE_NEW:
-		return load_late_stop_cpus();
+		return load_late_stop_cpus(false);
+	case UCODE_NEW_SAFE:
+		return load_late_stop_cpus(true);
 	case UCODE_NFOUND:
 		return -ENOENT;
 	default:
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -470,6 +470,9 @@ static enum ucode_state parse_microcode_
 	unsigned int curr_mc_size = 0;
 	u8 *new_mc = NULL, *mc = NULL;
 
+	if (force_minrev)
+		return UCODE_NFOUND;
+
 	while (iov_iter_count(iter)) {
 		struct microcode_header_intel mc_header;
 		unsigned int mc_size, data_size;
--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/arch/x86/kernel/cpu/microcode/internal.h
@@ -13,6 +13,7 @@ struct device;
 enum ucode_state {
 	UCODE_OK	= 0,
 	UCODE_NEW,
+	UCODE_NEW_SAFE,
 	UCODE_UPDATED,
 	UCODE_NFOUND,
 	UCODE_ERROR,
@@ -36,6 +37,8 @@ struct microcode_ops {
 				use_nmi		: 1;
 };
 
+extern bool force_minrev;
+
 extern struct ucode_cpu_info ucode_cpu_info[];
 struct cpio_data find_microcode_in_initrd(const char *path);
 

