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Message-ID: <20250827195753.GB3572128@ax162>
Date: Wed, 27 Aug 2025 12:57:53 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.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>, 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: Re: [PATCH 3/5] x86/cfi: Add option for cfi=debug bootparam
On Mon, Aug 25, 2025 at 07:25:50AM -0700, Kees Cook wrote:
> 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>
I am not sure if the x86 maintainers are "patch count adverse" but it
feels like this would be a little easier to review as four separate
patches. Every sentence in the commit message is basically its own
change.
1. The initial documentation for cfi= and its current values.
2. Standardization of pr_info() calls to use "CFI:"
3. Adding "CFI: Using" to __apply_fineibt()
4. Adding cfi=debug
Patch four would become much simpler to understand, especially with
Peter's suggested change.
> ---
> .../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);
You lowercase "Ignoring" earlier but not here, intentional? There are a
couple of other messages that have a capital first letter but not
others.
> }
>
> 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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