[PATCH 2/5] x86/microcode/intel: Check against CPU signature before saving microcode

[Ashok Raj]

When save_microcode_patch() is looking to replace an existing microcode in
the cache, current code is *only* checks the CPU sig/pf in the main
header. Microcode can carry additional sig/pf combinations in the extended
signature table, which is completely missed today.

For e.g. Current patch is a multi-stepping patch and new incoming patch is
a specific patch just for this CPUs stepping.

patch1:
fms3 <--- header FMS
...
ext_sig:
fms1
fms2

patch2: new
fms2 <--- header FMS

Current code takes only fms3 and checks with patch2 fms2.

saved_patch.header.fms3 != new_patch.header.fms2, so save_microcode_patch
saves it to the end of list instead of replacing patch1 with patch2.

There is no functional user observable issue since find_patch() skips
patch versions that are <= current_patch and will land on patch2 properly.

Nevertheless this will just end up storing every patch that isn't required.
Kernel just needs to store the latest patch. Otherwise its a memory leak
that sits in kernel and never used.

Tested-by: William Xie <william.xie@xxxxxxxxx>
Reported-by: William Xie <william.xie@xxxxxxxxx>
Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
---
 arch/x86/kernel/cpu/microcode/intel.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 025c8f0cd948..c4b11e2fbe33 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -114,10 +114,18 @@ static void save_microcode_patch(struct ucode_cpu_info *uci, void *data, unsigne
 
 	list_for_each_entry_safe(iter, tmp, &microcode_cache, plist) {
 		mc_saved_hdr = (struct microcode_header_intel *)iter->data;
-		sig	     = mc_saved_hdr->sig;
-		pf	     = mc_saved_hdr->pf;
 
-		if (find_matching_signature(data, sig, pf)) {
+		sig = uci->cpu_sig.sig;
+		pf  = uci->cpu_sig.pf;
+
+		/*
+		 * Compare the current CPUs signature with the ones in the
+		 * cache to identify the right candidate to replace. At any
+		 * given time, we should have no more than one valid patch
+		 * file for a given CPU fms+pf in the cache list.
+		 */
+
+		if (find_matching_signature(iter->data, sig, pf)) {
 			prev_found = true;
 
 			if (mc_hdr->rev <= mc_saved_hdr->rev)
-- 
2.32.0