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Subject[PATCH RFC 2/3] x86: microcode: intel: process microcode metadata
Date
Process the microcode metadata to see what changes are done on
CPU feature bits.

The default kernel policy imposed by is_microcode_allowed() is
to abort any microcode late loading if there will be removed a
feature and will continue the loading otherwise. Complex policies
can be created in the future.

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
---
arch/x86/kernel/cpu/microcode/intel.c | 85 +++++++++++++++++++++++++++++++++++
1 file changed, 85 insertions(+)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index a54d5e6..2ef4338 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -23,6 +23,7 @@
#include <linux/firmware.h>
#include <linux/uaccess.h>
#include <linux/vmalloc.h>
+#include <linux/string.h>
#include <linux/initrd.h>
#include <linux/kernel.h>
#include <linux/slab.h>
@@ -965,6 +966,81 @@ static bool is_blacklisted(unsigned int cpu)

return false;
}
+/**
+ * is_microcode_allowed - parse and enforce policies
+ * @firmware - metadata blob
+ *
+ * Parses the metadata file provided with the microcode in order to see the
+ * changes brought in by the new blob.
+ *
+ * Based on the type of changes, currently it enforces the following policy: if
+ * there is a feature being removed by the new microcode, reject the loading.
+ *
+ * RETURNS:
+ * false if it rejects the microcode or true if it accepts the microcode.
+ */
+static bool is_microcode_allowed(const struct firmware *firmware)
+{
+ int ret, index, leaf, eax, ebx, ecx, edx, line_nr = 0;
+ char *data = (char *) firmware->data;
+ char *line;
+ char op;
+
+ /*
+ * Get each line and match it with regular expression:
+ * {m|c} {+|-} u32 [u32]*
+ *
+ * See section Late loading metadata file from
+ * Documentation/x86/microcode.rst
+ */
+ while (data && (data - ((char *)(firmware->data)) < firmware->size)) {
+ line = strsep(&data, "\n");
+ line_nr++;
+
+ /*
+ * For each line check the first letter to see the type of
+ * operation done by the microcode and create a sscanf call
+ * the would match the rest of the line of that operation.
+ * If it is not a match, exist with parsing error. If it is
+ * a match and is a remove action (-) do not allow microcode
+ * to be loaded.
+ */
+ switch (line[0]) {
+ case 'c':
+ ret = sscanf(&line[1], " %c %x %x %x %x %x %x", &op,
+ &index, &leaf, &eax, &ebx, &ecx, &edx);
+ if (ret != 7)
+ goto out_error;
+ if (op == '-')
+ goto out_cpuid_rem_notsupp;
+ break;
+ case 'm':
+ ret = sscanf(&line[1], " %c %x", &op, &index);
+ if (ret != 2)
+ goto out_error;
+ if (op == '-')
+ goto out_msr_rem_notsupp;
+ break;
+ default:
+ goto out_error;
+ }
+ }
+
+ return true;
+
+out_error:
+ pr_warn("Microcode metadata failed parsing at line %d\n", line_nr);
+ return false;
+
+out_msr_rem_notsupp:
+ pr_warn("Kernel policy does not allow to remove MSR: %x\n", index);
+ return false;
+
+out_cpuid_rem_notsupp:
+ pr_warn("Kernel policy does not allow to remove CPUID: %x leaf: %x, eax: %x, ebx: %x, ecx: %x, edx: %x\n",
+ index, leaf, eax, ebx, ecx, edx);
+ return false;
+}

static enum ucode_state request_microcode_fw(int cpu, struct device *device,
bool refresh_fw)
@@ -992,10 +1068,19 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
return UCODE_NFOUND;
}

+ /* Load the metadata file in memory. */
if (request_firmware_direct(&meta_firmware, meta_name, device)) {
pr_debug("metadata file %s load failed\n", name);
pr_debug("no feature check will be done pre-loading the microcode\n");
} else {
+ /* Check the policy based on the metadata file. */
+ if (!is_microcode_allowed(meta_firmware)) {
+ pr_warn("kernel does not support the new microcode: %s\n",
+ name);
+ release_firmware(meta_firmware);
+ release_firmware(firmware);
+ return UCODE_ERROR;
+ }
release_firmware(meta_firmware);
}

--
1.8.3.1
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