x86/srso: Fix return thunks in generated code

Set X86_FEATURE_RETHUNK when enabling the SRSO mitigation so that
generated code (e.g., ftrace, static call, eBPF) generates "jmp
__x86_return_thunk" instead of RET.

  [ bp: Add a comment. ]

Fixes: fb3bd914 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>

arch/x86/kernel/alternative.c

@@ -707,9 +707,7 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes)
 	int i = 0;
 
 	/* Patch the custom return thunks... */
-	if (cpu_feature_enabled(X86_FEATURE_RETHUNK) ||
-	    cpu_feature_enabled(X86_FEATURE_SRSO) ||
-	    cpu_feature_enabled(X86_FEATURE_SRSO_ALIAS)) {
+	if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
 		i = JMP32_INSN_SIZE;
 		__text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i);
 	} else {

arch/x86/kernel/cpu/bugs.c

@@ -2297,6 +2297,12 @@ static void __init srso_select_mitigation(void)
 
 	case SRSO_CMD_SAFE_RET:
 		if (IS_ENABLED(CONFIG_CPU_SRSO)) {
+			/*
+			 * Enable the return thunk for generated code
+			 * like ftrace, static_call, etc.
+			 */
+			setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+
 			if (boot_cpu_data.x86 == 0x19)
 				setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
 			else