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Subject[PATCH 3/3] module: Support to disable validity enforcement in runtime
Date
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 > no_sig_enforce
$ openssl smime -sign -nocerts -noattr -binary \
-in no_sig_enforce -inkey <system_trusted_key> \
-signer <cert> -outform der -out no_sig_enforce.p7s
$ sudo cat no_sig_enforce.p7s \
> /sys/kernel/security/modsign/disable_enforce

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.jia@linux.alibaba.com>
---
kernel/module.c | 140 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 140 insertions(+)

diff --git a/kernel/module.c b/kernel/module.c
index 79825ea..3dd35ac 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);
@@ -2794,11 +2800,139 @@ static int module_sig_check(struct load_info *info, int flags)

return err;
}
+
+#ifdef CONFIG_SECURITYFS
+/*
+ * Check the input for disabling enforcement policy.
+ *
+ * Return 0 if inteding to disabling the policy. Note that the root
+ * privilege cannot simply disable the policy without the
+ * authentication given by a trusted key.
+ */
+static int check_disable_enforce(char *buf, size_t count)
+{
+ u8 *p;
+
+ /*
+ * 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_disable_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_disable_enforce(buf, count);
+ if (!ret) {
+ if (is_module_sig_enforced()) {
+ ret = disable_enforce(buf, count);
+ if (!ret) {
+ set_module_sig_enforce(false);
+ pr_notice("Kernel module validity enforcement disabled\n");
+ ret = count;
+ }
+ } else
+ ret = count;
+ }
+
+ kfree(buf);
+
+ return ret;
+}
+
+static const struct file_operations modsign_disable_enforce_ops = {
+ .write = modsign_disable_enforce_write,
+ .llseek = generic_file_llseek,
+};
+
+static int __init securityfs_init(void)
+{
+ struct dentry *modsign_dir;
+ struct dentry *disable_enforce;
+
+ modsign_dir = securityfs_create_dir("modsign", NULL);
+ if (IS_ERR(modsign_dir))
+ return -1;
+
+ disable_enforce = securityfs_create_file("disable_enforce", S_IWUSR,
+ modsign_dir, NULL,
+ &modsign_disable_enforce_ops);
+ if (IS_ERR(disable_enforce))
+ goto out;
+
+ return 0;
+out:
+ securityfs_remove(modsign_dir);
+
+ return -1;
+}
+#else /* !CONFIG_SECURITYFS */
+static int __init securityfs_init(void)
+{
+ return 0;
+}
+#endif
#else /* !CONFIG_MODULE_SIG */
static int module_sig_check(struct load_info *info, int flags)
{
return 0;
}
+
+static int __init securityfs_init(void)
+{
+ return 0;
+}
#endif /* !CONFIG_MODULE_SIG */

/* Sanity checks against invalid binaries, wrong arch, weird elf version. */
@@ -4395,8 +4529,14 @@ void module_layout(struct module *mod,

static int __init initialize_module(void)
{
+ int ret;
+
proc_modules_init();

+ ret = securityfs_init();
+ if (unlikely(ret))
+ return ret;
+
return 0;
}
module_init(initialize_module);
--
1.8.3.1
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