x86/bpf: Use alternative RET encoding

Use the return thunk in eBPF generated code, if needed.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>

arch/x86/net/bpf_jit_comp.c

···
 	*pprog = prog;
 }
 
+static void emit_return(u8 **pprog, u8 *ip)
+{
+	u8 *prog = *pprog;
+
+	if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
+		emit_jump(&prog, &__x86_return_thunk, ip);
+	} else {
+		EMIT1(0xC3);		/* ret */
+		if (IS_ENABLED(CONFIG_SLS))
+			EMIT1(0xCC);	/* int3 */
+	}
+
+	*pprog = prog;
+}
+
 /*
  * Generate the following code:
  *
···
 			ctx->cleanup_addr = proglen;
 			pop_callee_regs(&prog, callee_regs_used);
 			EMIT1(0xC9);         /* leave */
-			EMIT1(0xC3);         /* ret */
+			emit_return(&prog, image + addrs[i - 1] + (prog - temp));
 			break;
 
 		default:
···
 	if (flags & BPF_TRAMP_F_SKIP_FRAME)
 		/* skip our return address and return to parent */
 		EMIT4(0x48, 0x83, 0xC4, 8); /* add rsp, 8 */
-	EMIT1(0xC3); /* ret */
+	emit_return(&prog, prog);
 	/* Make sure the trampoline generation logic doesn't overflow */
 	if (WARN_ON_ONCE(prog > (u8 *)image_end - BPF_INSN_SAFETY)) {
 		ret = -EFAULT;