In order to disable the module validity enforcement, writing
a PKCS#7 signature corresponding the signed content '0' is
required. Given a simple way to archive this:

$ echo -n 0 > data
$ openssl smime -sign -nocerts -noattr -binary -in data \
    -inkey <system_trusted_key> -signer <cert> -outform der \
    -out data.sig

Note that the signing key must be a trust key located in
system trusted keyring. So even the root privilige cannot
simply disable the enforcement.

Signed-off-by: Jia Zhang <zhang....@linux.alibaba.com>
---
 kernel/module.c | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 114 insertions(+), 4 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 6b032577..16be198 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -64,6 +64,7 @@
 #include <linux/bsearch.h>
 #include <linux/dynamic_debug.h>
 #include <linux/audit.h>
+#include <linux/verification.h>
 #include <uapi/linux/module.h>
 #include "module-internal.h"
 
@@ -288,6 +289,11 @@ bool is_module_sig_enforced(void)
 }
 EXPORT_SYMBOL(is_module_sig_enforced);
 
+static void set_module_sig_enforce(bool enforce)
+{
+       sig_enforce = enforce;
+}
+
 /* Block module loading/unloading? */
 int modules_disabled = 0;
 core_param(nomodule, modules_disabled, bint, 0);
@@ -2796,6 +2802,61 @@ static int module_sig_check(struct load_info *info, int 
flags)
 }
 
 #ifdef CONFIG_SECURITYFS
+/*
+ * Check the intention of setting the enforcement policy.
+ *
+ * Return 1 if enabling the policy, or return 0 if disabling
+ * the policy. Note that the root privilege cannot simply
+ * disable the policy without the authentication given by a
+ * trusted key.
+ */
+static int check_enforce(char *buf, size_t count)
+{
+       u8 *p;
+
+       if (buf[0] == '1') {
+               if (count == 1 || (count == 2 && buf[1] == '\n'))
+                       return 1;
+
+               return -EINVAL;
+       }
+
+       /*
+        * In order to disable the enforcement policy, a PKCS#7 signature
+        * is supplied.
+        *
+        * Assuming ASN.1 encoding supplied, the minimal length would be
+        * 4-byte header plus at least 256-byte payload.
+        */
+       if (count < 260)
+               return -EINVAL;
+
+       p = (u8 *)buf;
+
+       /* The primitive type must be a sequnce */
+       if (p[0] != 0x30 || p[1] != 0x82)
+               return -EINVAL;
+
+       /* Match up the length of the supplied buffer */
+       if (be16_to_cpup((__be16 *)(p + 2)) != count - 4)
+               return -EINVAL;
+
+       return 0;
+}
+
+/*
+ * Disable the enforceme and verify the supplied PKCS#7 signature.
+ * The signed content is simply the charactoror '0'.
+ */
+static int disable_enforce(void *pkcs7, size_t pkcs7_len)
+{
+       char data = '0';
+
+       return verify_pkcs7_signature(&data, sizeof(data), pkcs7, pkcs7_len,
+                                     NULL, VERIFYING_UNSPECIFIED_SIGNATURE,
+                                     NULL, NULL);
+}
+
 static ssize_t modsign_enforce_read(struct file *filp, char __user *ubuf,
                                    size_t count, loff_t *offp)
 {
@@ -2806,7 +2867,50 @@ static ssize_t modsign_enforce_read(struct file *filp, 
char __user *ubuf,
        return simple_read_from_buffer(ubuf, count, offp, buf, 1);
 }
 
-static const struct file_operations modsign_enforce_ops = {
+static ssize_t modsign_enforce_write(struct file *filp,
+                                    const char __user *ubuf,
+                                    size_t count, loff_t *offp)
+{
+       char *buf;
+       ssize_t ret;
+       size_t max_buf_size = 1 << MAX_ORDER;
+
+       if (*offp > 1)
+               return -EFBIG;
+
+       if (count > max_buf_size)
+               return -EFBIG;
+
+       buf = kmalloc(count, GFP_KERNEL);
+       if (!buf)
+               return -ENOMEM;
+
+       ret = simple_write_to_buffer(buf, count, offp, ubuf, count);
+       if (ret <= 0) {
+               kfree(buf);
+               return ret;
+       }
+
+       ret = check_enforce(buf, count);
+       if (is_module_sig_enforced() && !ret) {
+               ret = disable_enforce(buf, count);
+               if (!ret) {
+                       set_module_sig_enforce(false);
+                       pr_notice("Kernel module validity enforcement 
disabled\n");
+                       ret = count;
+               }
+       } else if (!is_module_sig_enforced() && ret == 1) {
+               set_module_sig_enforce(true);
+               pr_notice("Kernel module validity enforcement enabled\n");
+               ret = count;
+       }
+
+       kfree(buf);
+
+       return ret;
+}
+
+static struct file_operations modsign_enforce_ops = {
        .read = modsign_enforce_read,
        .llseek = generic_file_llseek,
 };
@@ -2815,14 +2919,20 @@ static int __init securityfs_init(void)
 {
        struct dentry *modsign_dir;
        struct dentry *enforce;
+       umode_t mode;
 
        modsign_dir = securityfs_create_dir("modsign", NULL);
        if (IS_ERR(modsign_dir))
                return -1;
 
-       enforce = securityfs_create_file("enforce",
-                                        S_IRUSR | S_IRGRP, modsign_dir,
-                                        NULL, &modsign_enforce_ops);
+       mode = S_IRUSR | S_IRGRP;
+       if (!is_module_sig_enforced()) {
+               modsign_enforce_ops.write = modsign_enforce_write;
+               mode |= S_IWUSR;
+       }
+
+       enforce = securityfs_create_file("enforce", mode, modsign_dir, NULL,
+                                        &modsign_enforce_ops);
        if (IS_ERR(enforce))
                goto out;
 
-- 
1.8.3.1

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