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Date:   Tue, 18 Oct 2022 21:56:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     x86@kernel.org, Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
Message-ID: <Y08E9DgGD0lPB85O@hirez.programming.kicks-ass.net>
References: <Y06rtoE9BsERG9uv@hirez.programming.kicks-ass.net>
 <202210181020.79AF7F7@keescook>
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Precedence: bulk

On Tue, Oct 18, 2022 at 11:09:13AM -0700, Kees Cook wrote:
> > +config FINEIBT
> > +	def_bool y
> > +	depends on X86_KERNEL_IBT && CFI_CLANG
> > +	select CALL_PADDING
> 
> To that end, can we please make this a prompted choice?

How about something like so instead?

---
Subject: x86/cfi: Boot time selection of CFI scheme
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Oct 18 21:50:54 CEST 2022

Add the "cfi=" boot parameter to allow users to select a scheme at
boot time.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/alternative.c |  103 +++++++++++++++++++++++++++++++++---------
 1 file changed, 83 insertions(+), 20 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -702,6 +702,47 @@ void __init_or_module noinline apply_ibt
 #endif /* CONFIG_X86_KERNEL_IBT */
 
 #ifdef CONFIG_FINEIBT
+
+enum cfi_mode {
+	CFI_DEFAULT,
+	CFI_OFF,
+	CFI_KCFI,
+	CFI_FINEIBT,
+};
+
+static enum cfi_mode cfi_mode __ro_after_init = CFI_DEFAULT;
+
+static __init int cfi_parse_cmdline(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	while (str) {
+		char *next = strchr(str, ',');
+		if (next) {
+			*next = 0;
+			next++;
+		}
+
+		if (!strcmp(str, "auto")) {
+			cfi_mode = CFI_DEFAULT;
+		} else if (!strcmp(str, "off")) {
+			cfi_mode = CFI_OFF;
+		} else if (!strcmp(str, "kcfi")) {
+			cfi_mode = CFI_KCFI;
+		} else if (!strcmp(str, "fineibt")) {
+			cfi_mode = CFI_FINEIBT;
+		} else {
+			pr_err("Ignoring unknown cfi option (%s).", str);
+		}
+
+		str = next;
+	}
+
+	return 0;
+}
+early_param("cfi", cfi_parse_cmdline);
+
 /*
  * kCFI						FineIBT
  *
@@ -868,30 +909,52 @@ static void __apply_fineibt(s32 *start_r
 		      "FineIBT preamble wrong size: %ld", fineibt_preamble_size))
 		return;
 
-	if (!HAS_KERNEL_IBT || !cpu_feature_enabled(X86_FEATURE_IBT))
+	if (cfi_mode == CFI_DEFAULT) {
+		cfi_mode = CFI_KCFI;
+		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))
+			cfi_mode = CFI_FINEIBT;
+	}
+
+	switch (cfi_mode) {
+	case CFI_OFF:
+		ret = cfi_disable_callers(start_retpoline, end_retpoline);
+		if (ret)
+			goto err;
+
+		if (builtin)
+			pr_info("Disabling CFI\n");
 		return;
 
-	/*
-	 * Rewrite the callers to not use the __cfi_ stubs, such that we might
-	 * rewrite them. This disables all CFI. If this succeeds but any of the
-	 * later stages fails, we're without CFI.
-	 */
-	ret = cfi_disable_callers(start_retpoline, end_retpoline);
-	if (ret)
-		goto err;
-
-	ret = cfi_rewrite_preamble(start_cfi, end_cfi);
-	if (ret)
-		goto err;
-
-	ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
-	if (ret)
-		goto err;
+	case CFI_KCFI:
+		if (builtin)
+			pr_info("Using kCFI\n");
+		return;
 
-	if (builtin)
-		pr_info("Using FineIBT CFI\n");
+	case CFI_FINEIBT:
+		/*
+		 * Rewrite the callers to not use the __cfi_ stubs, such that we might
+		 * rewrite them. This disables all CFI. If this succeeds but any of the
+		 * later stages fails, we're without CFI.
+		 */
+		ret = cfi_disable_callers(start_retpoline, end_retpoline);
+		if (ret)
+			goto err;
+
+		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
+		if (ret)
+			goto err;
+
+		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
+		if (ret)
+			goto err;
 
-	return;
+		if (builtin)
+			pr_info("Using FineIBT CFI\n");
+		return;
+
+	default:
+		break;
+	}
 
 err:
 	pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");