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Message-ID: <20221027092842.699804264@infradead.org>
Date: Thu, 27 Oct 2022 11:28:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
Kees Cook <keescook@...omium.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
Joao Moreira <joao@...rdrivepizza.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Mark Rutland <mark.rutland@....com>
Subject: [PATCH 3/4] x86/cfi: Boot time selection of CFI scheme
Add the "cfi=" boot parameter to allow people to select a CFI scheme
at boot time. Mostly useful for development / debugging.
Requested-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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");
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