x86/microcode/AMD: Handle the case of no BIOS microcode

commit fcf8239 upstream.

Machines can be shipped without any microcode in the BIOS. Which means,
the microcode patch revision is 0.

Handle that gracefully.

Fixes: 94838d2 ("x86/microcode/AMD: Use the family,model,stepping encoded in the patch ID")
Reported-by: Vítek Vávra <vit.vavra.kh@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 43be33b8a2f2bab9c110ea08aded85ec4507ee7b)

Author: bp3tk0v

arch/x86/kernel/cpu/microcode/amd.c

@@ -159,8 +159,28 @@ static int cmp_id(const void *key, const void *elem)
 		return 1;
 }
 
+static u32 cpuid_to_ucode_rev(unsigned int val)
+{
+	union zen_patch_rev p = {};
+	union cpuid_1_eax c;
+
+	c.full = val;
+
+	p.stepping  = c.stepping;
+	p.model     = c.model;
+	p.ext_model = c.ext_model;
+	p.ext_fam   = c.ext_fam;
+
+	return p.ucode_rev;
+}
+
 static bool need_sha_check(u32 cur_rev)
 {
+	if (!cur_rev) {
+		cur_rev = cpuid_to_ucode_rev(bsp_cpuid_1_eax);
+		pr_info_once("No current revision, generating the lowest one: 0x%x\n", cur_rev);
+	}
+
 	switch (cur_rev >> 8) {
 	case 0x80012: return cur_rev <= 0x800126f; break;
 	case 0x80082: return cur_rev <= 0x800820f; break;
@@ -741,8 +761,6 @@ static struct ucode_patch *cache_find_patch(struct ucode_cpu_info *uci, u16 equi
 	n.equiv_cpu = equiv_cpu;
 	n.patch_id  = uci->cpu_sig.rev;
 
-	WARN_ON_ONCE(!n.patch_id);
-
 	list_for_each_entry(p, &microcode_cache, plist)
 		if (patch_cpus_equivalent(p, &n, false))
 			return p;