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Message-ID: <175757609542.709179.13938862888547182704.tip-bot2@tip-bot2>
Date: Thu, 11 Sep 2025 07:34:55 -0000
From: "tip-bot2 for Kees Cook" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Kees Cook <kees@...nel.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86/cfi: Add "debug" option to "cfi=" bootparam
The following commit has been merged into the x86/core branch of tip:
Commit-ID: 026211c40b055485b4c65c10d644a92864623225
Gitweb: https://git.kernel.org/tip/026211c40b055485b4c65c10d644a92864623225
Author: Kees Cook <kees@...nel.org>
AuthorDate: Wed, 03 Sep 2025 20:46:44 -07:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 04 Sep 2025 21:59:08 +02:00
x86/cfi: Add "debug" option to "cfi=" bootparam
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@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20250904034656.3670313-5-kees@kernel.org
---
Documentation/admin-guide/kernel-parameters.txt | 1 +-
arch/x86/kernel/alternative.c | 16 ++++++++++++++++-
2 files changed, 17 insertions(+)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8bbffbb..c8337d0 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 d8f4ac9..7d4a992 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")) {
@@ -1707,6 +1710,8 @@ static int cfi_rewrite_callers(s32 *start, s32 *end)
return 0;
}
+#define pr_cfi_debug(X...) if (cfi_debug) pr_info(X)
+
static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi, bool builtin)
{
@@ -1734,6 +1739,7 @@ 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.
*/
+ pr_cfi_debug("CFI: disabling all indirect call checking\n");
ret = cfi_disable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
@@ -1744,14 +1750,19 @@ 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);
}
+ 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;
+ pr_cfi_debug("CFI: rehashing all indirect calls\n");
ret = cfi_rand_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
+ } else {
+ pr_cfi_debug("CFI: rehashing disabled\n");
}
switch (cfi_mode) {
@@ -1761,6 +1772,7 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
return;
case CFI_KCFI:
+ pr_cfi_debug("CFI: re-enabling all indirect call checking\n");
ret = cfi_enable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
@@ -1771,17 +1783,20 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
return;
case CFI_FINEIBT:
+ pr_cfi_debug("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 */
+ pr_cfi_debug("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 */
+ pr_cfi_debug("CFI: removing old endbr insns\n");
cfi_rewrite_endbr(start_cfi, end_cfi);
if (builtin) {
@@ -2324,6 +2339,7 @@ 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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