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Message-Id: <20250825142603.1907143-3-kees@kernel.org>
Date: Mon, 25 Aug 2025 07:25:50 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Kees Cook <kees@...nel.org>,
Kees Cook <kees@...flux.net>,
Sami Tolvanen <samitolvanen@...gle.com>,
David Woodhouse <dwmw2@...radead.org>,
Linus Walleij <linus.walleij@...aro.org>,
Mark Rutland <mark.rutland@....com>,
Puranjay Mohan <puranjay@...nel.org>,
Jonathan Corbet <corbet@....net>,
Nathan Chancellor <nathan@...nel.org>,
x86@...nel.org,
linux-doc@...r.kernel.org,
linux-kbuild@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
linux-riscv@...ts.infradead.org,
llvm@...ts.linux.dev,
linux-hardening@...r.kernel.org
Subject: [PATCH 3/5] x86/cfi: Add option for cfi=debug bootparam
From: Kees Cook <kees@...flux.net>
Add "debug" option for "cfi=" bootparam to get details on early CFI
initialization steps. Standardize CFI pr_info() lines to use "CFI:"
prefix. Standardize "CFI: Using ..." to always report which CFI mode is
being used, regardless of CONFIG_FINEIBT. Document all the "cfi=" options.
Signed-off-by: Kees Cook <kees@...flux.net>
---
.../admin-guide/kernel-parameters.txt | 18 +++++++++
arch/x86/kernel/alternative.c | 39 +++++++++++++++----
2 files changed, 50 insertions(+), 7 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 747a55abf494..7b4bddb5a030 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -608,6 +608,24 @@
ccw_timeout_log [S390]
See Documentation/arch/s390/common_io.rst for details.
+ cfi= [X86-64] Set Control Flow Integrity checking features
+ when CONFIG_FINEIBT is enabled.
+ Format: feature[,feature...]
+ Default: auto
+
+ auto: Use FineIBT if IBT available, otherwise kCFI.
+ Under FineIBT, enable "paranoid" mode when
+ FRED is not available.
+ off: Turn off CFI checking.
+ kcfi: Use kCFI (disable FineIBT).
+ fineibt: Use FineIBT (even if IBT not available).
+ norand: Do not re-randomize CFI hashes.
+ paranoid: Add caller hash checking under FineIBT.
+ bhi: Enable register poisoning to stop speculation
+ across FineIBT. (Disabled by default.)
+ warn: Do not enforce CFI checking: warn only.
+ debug: Report CFI initialization details.
+
cgroup_disable= [KNL] Disable a particular controller or optional feature
Format: {name of the controller(s) or feature(s) to disable}
The effects of cgroup_disable=foo are:
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 7bde68247b5f..5d80ae77c042 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1225,6 +1225,7 @@ int cfi_get_func_arity(void *func)
static bool cfi_rand __ro_after_init = true;
static u32 cfi_seed __ro_after_init;
+static bool cfi_debug __ro_after_init;
/*
* Re-hash the CFI hash with a boot-time seed while making sure the result is
@@ -1259,6 +1260,8 @@ static __init int cfi_parse_cmdline(char *str)
} else if (!strcmp(str, "off")) {
cfi_mode = CFI_OFF;
cfi_rand = false;
+ } else if (!strcmp(str, "debug")) {
+ cfi_debug = true;
} else if (!strcmp(str, "kcfi")) {
cfi_mode = CFI_KCFI;
} else if (!strcmp(str, "fineibt")) {
@@ -1266,26 +1269,26 @@ static __init int cfi_parse_cmdline(char *str)
} else if (!strcmp(str, "norand")) {
cfi_rand = false;
} else if (!strcmp(str, "warn")) {
- pr_alert("CFI mismatch non-fatal!\n");
+ pr_alert("CFI: mismatch non-fatal!\n");
cfi_warn = true;
} else if (!strcmp(str, "paranoid")) {
if (cfi_mode == CFI_FINEIBT) {
cfi_paranoid = true;
} else {
- pr_err("Ignoring paranoid; depends on fineibt.\n");
+ pr_err("CFI: ignoring paranoid; depends on fineibt.\n");
}
} else if (!strcmp(str, "bhi")) {
#ifdef CONFIG_FINEIBT_BHI
if (cfi_mode == CFI_FINEIBT) {
cfi_bhi = true;
} else {
- pr_err("Ignoring bhi; depends on fineibt.\n");
+ pr_err("CFI: ignoring bhi; depends on fineibt.\n");
}
#else
- pr_err("Ignoring bhi; depends on FINEIBT_BHI=y.\n");
+ pr_err("CFI: ignoring bhi; depends on FINEIBT_BHI=y.\n");
#endif
} else {
- pr_err("Ignoring unknown cfi option (%s).", str);
+ pr_err("CFI: Ignoring unknown option (%s).", str);
}
str = next;
@@ -1734,6 +1737,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
* rewrite them. This disables all CFI. If this succeeds but any of the
* later stages fails, we're without CFI.
*/
+ if (builtin && cfi_debug)
+ pr_info("CFI: disabling all indirect call checking\n");
ret = cfi_disable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
@@ -1744,43 +1749,61 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
cfi_bpf_hash = cfi_rehash(cfi_bpf_hash);
cfi_bpf_subprog_hash = cfi_rehash(cfi_bpf_subprog_hash);
}
+ if (builtin && cfi_debug)
+ pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
+ if (builtin && cfi_debug)
+ pr_info("CFI: rehashing all preambles\n");
ret = cfi_rand_preamble(start_cfi, end_cfi);
if (ret)
goto err;
+ if (builtin && cfi_debug)
+ pr_info("CFI: rehashing all indirect calls\n");
ret = cfi_rand_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
+ } else {
+ if (builtin && cfi_debug)
+ pr_info("CFI: rehashing disabled\n");
}
switch (cfi_mode) {
case CFI_OFF:
if (builtin)
- pr_info("Disabling CFI\n");
+ pr_info("CFI: disabled\n");
return;
case CFI_KCFI:
+ if (builtin && cfi_debug)
+ pr_info("CFI: enabling all indirect call checking\n");
ret = cfi_enable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
if (builtin)
- pr_info("Using kCFI\n");
+ pr_info("CFI: Using %s kCFI\n",
+ cfi_rand ? "rehashed" : "retpoline");
return;
case CFI_FINEIBT:
+ if (builtin && cfi_debug)
+ pr_info("CFI: adding FineIBT to all preambles\n");
/* place the FineIBT preamble at func()-16 */
ret = cfi_rewrite_preamble(start_cfi, end_cfi);
if (ret)
goto err;
/* rewrite the callers to target func()-16 */
+ if (builtin && cfi_debug)
+ pr_info("CFI: rewriting indirect call sites to use FineIBT\n");
ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
/* now that nobody targets func()+0, remove ENDBR there */
+ if (builtin && cfi_debug)
+ pr_info("CFI: removing old endbr insns\n");
cfi_rewrite_endbr(start_cfi, end_cfi);
if (builtin) {
@@ -2005,6 +2028,8 @@ bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi, bool builtin)
{
+ if (builtin)
+ pr_info("CFI: Using standard kCFI\n");
}
#ifdef CONFIG_X86_KERNEL_IBT
--
2.34.1
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