[PATCH] loadpin: Implement custom proc_handler for enforce

[Joel Granados <joel.granados@kernel.org>]

The new proc_handler_loadpin returns -EINVAL when is_loadpin_writable is
false and the kernel var (enforce) is being written. Move
loadpin_sysctl_table to .rodata (by const qualifying it) as there is no
need to change the value of the extra1 entry.

Signed-off-by: Joel Granados <joel.granados@kernel.org>
---
Const qualifying ctl tables is part of the hardening effort in the linux
kernel.
---
 security/loadpin/loadpin.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index 273ffbd6defe1324d6688dec5f9fe6c9401283ed..f049c81b82a78265b6ae358bb2a814265cec9f16 100644
--- a/security/loadpin/loadpin.c
+++ b/security/loadpin/loadpin.c
@@ -53,18 +53,29 @@ static bool deny_reading_verity_digests;
 #endif
 
 #ifdef CONFIG_SYSCTL
-static struct ctl_table loadpin_sysctl_table[] = {
+static bool is_loadpin_writable;
+
+static int proc_handler_loadpin(const struct ctl_table *table, int dir,
+				void *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (!is_loadpin_writable && SYSCTL_USER_TO_KERN(dir))
+		return -EINVAL;
+	return proc_dointvec_minmax(table, dir, buffer, lenp, ppos);
+}
+
+static const struct ctl_table loadpin_sysctl_table[] = {
 	{
 		.procname       = "enforce",
 		.data           = &enforce,
 		.maxlen         = sizeof(int),
 		.mode           = 0644,
-		.proc_handler   = proc_dointvec_minmax,
-		.extra1         = SYSCTL_ONE,
+		.proc_handler   = proc_handler_loadpin,
+		.extra1         = SYSCTL_ZERO,
 		.extra2         = SYSCTL_ONE,
 	},
 };
 
+
 static void set_sysctl(bool is_writable)
 {
 	/*
@@ -72,9 +83,9 @@ static void set_sysctl(bool is_writable)
 	 * device, allow sysctl to change modes for testing.
 	 */
 	if (is_writable)
-		loadpin_sysctl_table[0].extra1 = SYSCTL_ZERO;
+		is_loadpin_writable = true;
 	else
-		loadpin_sysctl_table[0].extra1 = SYSCTL_ONE;
+		is_loadpin_writable = false;
 }
 #else
 static inline void set_sysctl(bool is_writable) { }

---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251215-jag-const_loadpin-761f052f76c4

Best regards,
-- 
Joel Granados <joel.granados@kernel.org>