x86/alternatives: Catch late X86_FEATURE modifiers

JIRA: https://issues.redhat.com/browse/RHEL-68940

commit ee89620
Author: Borislav Petkov (AMD) <bp@alien8.de>
Date:   Wed, 27 Mar 2024 16:43:14 +0100

    x86/alternatives: Catch late X86_FEATURE modifiers

    After alternatives have been patched, changes to the X86_FEATURE flags
    won't take effect and could potentially even be wrong.

    Warn about it.

    This is something which has been long overdue.

    Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Tested-by: Srikanth Aithal <sraithal@amd.com>
    Link: https://lore.kernel.org/r/20240327154317.29909-3-bp@alien8.de

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

arch/x86/include/asm/cpufeature.h

@@ -163,8 +163,12 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 extern void setup_clear_cpu_cap(unsigned int bit);
 extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
 
-#define setup_force_cpu_cap(bit) do { \
-	set_cpu_cap(&boot_cpu_data, bit);	\
+#define setup_force_cpu_cap(bit) do {			\
+							\
+	if (!boot_cpu_has(bit))				\
+		WARN_ON(alternatives_patched);		\
+							\
+	set_cpu_cap(&boot_cpu_data, bit);		\
 	set_bit(bit, (unsigned long *)cpu_caps_set);	\
 } while (0)
 

arch/x86/kernel/cpu/cpuid-deps.c

@@ -112,6 +112,9 @@ static void do_clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int feature)
 	if (WARN_ON(feature >= MAX_FEATURE_BITS))
 		return;
 
+	if (boot_cpu_has(feature))
+		WARN_ON(alternatives_patched);
+
 	clear_feature(c, feature);
 
 	/* Collect all features to disable, handling dependencies */