Add "debug" option for "cfi=" bootparam to get details on early CFI
initialization steps so future Kees can find breakage easier.
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Peter Zijlstra <peterz@infradead.org>
---
.../admin-guide/kernel-parameters.txt | 1 +
arch/x86/kernel/alternative.c | 23 +++++++++++++++++++
2 files changed, 24 insertions(+)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8bbffbb334ab..c8337d0e6ba3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -624,6 +624,7 @@
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}
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d8f4ac95b4df..b311e31a9056 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1177,6 +1177,7 @@ void __init_or_module apply_seal_endbr(s32 *start, s32 *end) { }
#endif
enum cfi_mode cfi_mode __ro_after_init = __CFI_DEFAULT;
+static bool cfi_debug __ro_after_init;
#ifdef CONFIG_FINEIBT_BHI
bool cfi_bhi __ro_after_init = false;
@@ -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")) {
@@ -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 (cfi_debug)
+ pr_info("CFI: disabling all indirect call checking\n");
ret = cfi_disable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
@@ -1744,14 +1749,23 @@ 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 (cfi_debug)
+ pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
+ if (cfi_debug)
+ pr_info("CFI: rehashing all preambles\n");
ret = cfi_rand_preamble(start_cfi, end_cfi);
if (ret)
goto err;
+ if (cfi_debug)
+ pr_info("CFI: rehashing all indirect calls\n");
ret = cfi_rand_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
+ } else {
+ if (cfi_debug)
+ pr_info("CFI: rehashing disabled\n");
}
switch (cfi_mode) {
@@ -1761,6 +1775,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
return;
case CFI_KCFI:
+ if (cfi_debug)
+ pr_info("CFI: enabling all indirect call checking\n");
ret = cfi_enable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
@@ -1771,17 +1787,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
return;
case CFI_FINEIBT:
+ if (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 (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 (cfi_debug)
+ pr_info("CFI: removing old endbr insns\n");
cfi_rewrite_endbr(start_cfi, end_cfi);
if (builtin) {
@@ -2324,6 +2346,7 @@ void __init alternative_instructions(void)
__apply_fineibt(__retpoline_sites, __retpoline_sites_end,
__cfi_sites, __cfi_sites_end, true);
+ cfi_debug = false;
/*
* Rewrite the retpolines, must be done before alternatives since
--
2.34.1On Wed, Sep 03, 2025 at 08:46:44PM -0700, Kees Cook wrote:
> @@ -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 (cfi_debug)
> + pr_info("CFI: disabling all indirect call checking\n");
> ret = cfi_disable_callers(start_retpoline, end_retpoline);
> if (ret)
> goto err;
> @@ -1744,14 +1749,23 @@ 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 (cfi_debug)
> + pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
>
> + if (cfi_debug)
> + pr_info("CFI: rehashing all preambles\n");
So this repeated if() bugs the heck out of me ... :-)
What would you prefer, this:
if (cfi_debug) {
pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
pr_info("CFI: rehashing all preambles\n");
}
or something like:
#define pr_cfi_debug(X...) if (cfi_debug) pr_info(X)
pr_cfi_debug("CFI: cfi_seed: 0x%08x\n", cfi_seed);
pr_cfi_debug("CFI: rehashing all preambles\n");
?
> ret = cfi_rand_preamble(start_cfi, end_cfi);
> if (ret)
> goto err;
>
> + if (cfi_debug)
> + pr_info("CFI: rehashing all indirect calls\n");
> ret = cfi_rand_callers(start_retpoline, end_retpoline);
> if (ret)
> goto err;
> + } else {
> + if (cfi_debug)
> + pr_info("CFI: rehashing disabled\n");
> }
>
> switch (cfi_mode) {
> @@ -1761,6 +1775,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
> return;
>
> case CFI_KCFI:
> + if (cfi_debug)
> + pr_info("CFI: enabling all indirect call checking\n");
This should be "CFI: re-enabling all..." I suppose, to better match the
earlier "CFI: disabling all ..." message.
> ret = cfi_enable_callers(start_retpoline, end_retpoline);
> if (ret)
> goto err;
> @@ -1771,17 +1787,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
> return;
>
> case CFI_FINEIBT:
> + if (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 (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 (cfi_debug)
> + pr_info("CFI: removing old endbr insns\n");
> cfi_rewrite_endbr(start_cfi, end_cfi);
>
> if (builtin) {
> @@ -2324,6 +2346,7 @@ void __init alternative_instructions(void)
>
> __apply_fineibt(__retpoline_sites, __retpoline_sites_end,
> __cfi_sites, __cfi_sites_end, true);
> + cfi_debug = false;
>
> /*
> * Rewrite the retpolines, must be done before alternatives since
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