[PATCH 3/5] x86/microcode/intel: Allow a late-load only if a min rev is specified

Ashok Raj
2022-08-13T22:39:06Z

In general users don't have the necessary information to determine
whether a late-load of a new microcode version has removed any feature
(MSR, CPUID etc) between what is currently loaded and this new microcode.
To address this issue, Intel has added a "minimum required version" field to
a previously reserved field in the file header. Microcode updates
should only be applied if the current microcode version is equal
to, or greater than this minimum required version.

https://lore.kernel.org/linux-kernel/alpine.DEB.2.21.1909062237580.1902@nanos.tec.linutronix.de/

Thomas made some suggestions on how meta-data in the microcode file could
provide Linux with information to decide if the new microcode is suitable
candidate for late-load. But even the "simpler" option#1 requires a lot of
metadata and corresponding kernel code to parse it.

The proposal here is an even simpler option. The criteria for a microcode to
be a viable late-load candidate is that no CPUID or OS visible MSR features
are removed with respect to an earlier version of the microcode.

Pseudocode for late-load is as follows:

if header.min_required_id == 0
This is old format microcode, block late-load
else if current_ucode_version < header.min_required_id
Current version is too old, block late-load of this microcode.
else
OK to proceed with late-load.

Any microcode that removes a feature will set the min_version to itself.
This will enforce this microcode is not suitable for late-loading.

The enforcement is not in hardware and limited to kernel loader enforcing
the requirement. It is not required for early loading of microcode to
enforce this requirement, since the new features are only
evaluated after early loading in the boot process.

Test cases covered:

1. With new kernel, attempting to load an older format microcode with the
min_rev=0 should be blocked by kernel.

[ 210.541802] microcode: Header MUST specify min version for late-load

2. New microcode with a non-zero min_rev in the header, but the specified
min_rev is greater than what is currently loaded in the CPU should be
blocked by kernel.

245.139828] microcode: Current revision 0x8f685300 is too old to update,
must be at 0xaa000050 version or higher

3. New microcode with a min_rev < currently loaded should allow loading the
microcode

4. Build initrd with microcode that has min_rev=0, or min_rev > currently
loaded should permit early loading microcode from initrd.

Tested-by: William Xie <william.xie@intel.com>
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
---
arch/x86/include/asm/microcode_intel.h | 4 +++-
arch/x86/kernel/cpu/microcode/intel.c | 20 ++++++++++++++++++++
2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/microcode_intel.h b/arch/x86/include/asm/microcode_intel.h
index 4c92cea7e4b5..16b8715e0984 100644
--- a/arch/x86/include/asm/microcode_intel.h
+++ b/arch/x86/include/asm/microcode_intel.h
@@ -14,7 +14,9 @@ struct microcode_header_intel {
unsigned int pf;
unsigned int datasize;
unsigned int totalsize;
- unsigned int reserved[3];
+ unsigned int reserved1;
+ unsigned int min_req_id;
+ unsigned int reserved3;
};

struct microcode_intel {
diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index c4b11e2fbe33..1eb202ec2302 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -178,6 +178,7 @@ static int microcode_sanity_check(void *mc, int print_err)
struct extended_sigtable *ext_header = NULL;
u32 sum, orig_sum, ext_sigcount = 0, i;
struct extended_signature *ext_sig;
+ struct ucode_cpu_info uci;

total_size = get_totalsize(mc_header);
data_size = get_datasize(mc_header);
@@ -248,6 +249,25 @@ static int microcode_sanity_check(void *mc, int print_err)
return -EINVAL;
}

+ /*
+ * Enforce for late-load that min_req_id is specified in the header.
+ * Otherwise its an old format microcode, reject it.
+ */
+ if (print_err) {
+ if (!mc_header->min_req_id) {
+ pr_warn("Header MUST specify min version for late-load\n");
+ return -EINVAL;
+ }
+
+ intel_cpu_collect_info(&uci);
+ if (uci.cpu_sig.rev < mc_header->min_req_id) {
+ pr_warn("Current revision 0x%x is too old to update,"
+ "must be at 0x%x version or higher\n",
+ uci.cpu_sig.rev, mc_header->min_req_id);
+ return -EINVAL;
+ }
+ }
+
if (!ext_table_size)
return 0;

--
2.32.0