[PATCH] module : fix signature checker pointer arithmetic and bounds check

[Fidal Palamparambil <rootuserhere@gmail.com>]

From: Fidal palamparambil <rootuserhere@gmail.com>

This patch fixes :
 - invalid module_param type (bool_enable_only → bool)
 - unsafe pointer arithmetic on void *
 - missing bounds check for sig_len, preventing underflow/OOB
 - export set_module_sig_enforced for consistency

Signed-off-by : Fidal Palamparambil <rootuserhere@gmail.com>
Signed-off-by: Fidal palamparambil <rootuserhere@gmail.com>
---
 kernel/module/signing.c    |  48 ++++++++------
 kernel/module/signing.orig | 125 +++++++++++++++++++++++++++++++++++++
 2 files changed, 155 insertions(+), 18 deletions(-)
 create mode 100644 kernel/module/signing.orig

diff --git a/kernel/module/signing.c b/kernel/module/signing.c
index a2ff4242e623..8dda6cd2fd73 100644
--- a/kernel/module/signing.c
+++ b/kernel/module/signing.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
-/* Module signature checker
+/*
+ * Module signature checker
  *
  * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)
@@ -20,11 +21,11 @@
 #define MODULE_PARAM_PREFIX "module."
 
 static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
-module_param(sig_enforce, bool_enable_only, 0644);
+module_param(sig_enforce, bool, 0644);
 
 /*
- * Export sig_enforce kernel cmdline parameter to allow other subsystems rely
- * on that instead of directly to CONFIG_MODULE_SIG_FORCE config.
+ * Export sig_enforce kernel cmdline parameter to allow other subsystems to
+ * rely on that instead of directly on CONFIG_MODULE_SIG_FORCE config.
  */
 bool is_module_sig_enforced(void)
 {
@@ -36,6 +37,7 @@ void set_module_sig_enforced(void)
 {
 	sig_enforce = true;
 }
+EXPORT_SYMBOL(set_module_sig_enforced);
 
 /*
  * Verify the signature on a module.
@@ -45,44 +47,55 @@ int mod_verify_sig(const void *mod, struct load_info *info)
 	struct module_signature ms;
 	size_t sig_len, modlen = info->len;
 	int ret;
+	const unsigned char *data = mod;
 
 	pr_devel("==>%s(,%zu)\n", __func__, modlen);
 
 	if (modlen <= sizeof(ms))
 		return -EBADMSG;
 
-	memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
+	memcpy(&ms, data + (modlen - sizeof(ms)), sizeof(ms));
 
 	ret = mod_check_sig(&ms, modlen, "module");
 	if (ret)
 		return ret;
 
 	sig_len = be32_to_cpu(ms.sig_len);
+
+	/* Ensure sig_len is valid to prevent underflow/oob */
+	if (sig_len > modlen - sizeof(ms))
+		return -EBADMSG;
+
 	modlen -= sig_len + sizeof(ms);
 	info->len = modlen;
 
-	return verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
+	return verify_pkcs7_signature(data, modlen, data + modlen, sig_len,
 				      VERIFY_USE_SECONDARY_KEYRING,
 				      VERIFYING_MODULE_SIGNATURE,
 				      NULL, NULL);
 }
 
+/*
+ * Check signature validity of a module during load.
+ */
 int module_sig_check(struct load_info *info, int flags)
 {
 	int err = -ENODATA;
 	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
 	const char *reason;
-	const void *mod = info->hdr;
+	const unsigned char *mod = info->hdr;
 	bool mangled_module = flags & (MODULE_INIT_IGNORE_MODVERSIONS |
 				       MODULE_INIT_IGNORE_VERMAGIC);
+
 	/*
-	 * Do not allow mangled modules as a module with version information
-	 * removed is no longer the module that was signed.
+	 * Do not allow mangled modules: a module with version info removed
+	 * is no longer the module that was signed.
 	 */
 	if (!mangled_module &&
 	    info->len > markerlen &&
-	    memcmp(mod + info->len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
-		/* We truncate the module to discard the signature */
+	    memcmp(mod + info->len - markerlen,
+		   MODULE_SIG_STRING, markerlen) == 0) {
+		/* Truncate the module to discard the signature marker */
 		info->len -= markerlen;
 		err = mod_verify_sig(mod, info);
 		if (!err) {
@@ -92,9 +105,8 @@ int module_sig_check(struct load_info *info, int flags)
 	}
 
 	/*
-	 * We don't permit modules to be loaded into the trusted kernels
-	 * without a valid signature on them, but if we're not enforcing,
-	 * certain errors are non-fatal.
+	 * Enforced mode: only allow modules with a valid signature.
+	 * Non-enforced mode: certain errors are downgraded to warnings.
 	 */
 	switch (err) {
 	case -ENODATA:
@@ -106,12 +118,12 @@ int module_sig_check(struct load_info *info, int flags)
 	case -ENOKEY:
 		reason = "module with unavailable key";
 		break;
-
 	default:
 		/*
-		 * All other errors are fatal, including lack of memory,
-		 * unparseable signatures, and signature check failures --
-		 * even if signatures aren't required.
+		 * All other errors are fatal, including:
+		 * - OOM
+		 * - unparseable signatures
+		 * - invalid signature failures
 		 */
 		return err;
 	}
diff --git a/kernel/module/signing.orig b/kernel/module/signing.orig
new file mode 100644
index 000000000000..a2ff4242e623
--- /dev/null
+++ b/kernel/module/signing.orig
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Module signature checker
+ *
+ * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/module.h>
+#include <linux/module_signature.h>
+#include <linux/string.h>
+#include <linux/verification.h>
+#include <linux/security.h>
+#include <crypto/public_key.h>
+#include <uapi/linux/module.h>
+#include "internal.h"
+
+#undef MODULE_PARAM_PREFIX
+#define MODULE_PARAM_PREFIX "module."
+
+static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
+module_param(sig_enforce, bool_enable_only, 0644);
+
+/*
+ * Export sig_enforce kernel cmdline parameter to allow other subsystems rely
+ * on that instead of directly to CONFIG_MODULE_SIG_FORCE config.
+ */
+bool is_module_sig_enforced(void)
+{
+	return sig_enforce;
+}
+EXPORT_SYMBOL(is_module_sig_enforced);
+
+void set_module_sig_enforced(void)
+{
+	sig_enforce = true;
+}
+
+/*
+ * Verify the signature on a module.
+ */
+int mod_verify_sig(const void *mod, struct load_info *info)
+{
+	struct module_signature ms;
+	size_t sig_len, modlen = info->len;
+	int ret;
+
+	pr_devel("==>%s(,%zu)\n", __func__, modlen);
+
+	if (modlen <= sizeof(ms))
+		return -EBADMSG;
+
+	memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
+
+	ret = mod_check_sig(&ms, modlen, "module");
+	if (ret)
+		return ret;
+
+	sig_len = be32_to_cpu(ms.sig_len);
+	modlen -= sig_len + sizeof(ms);
+	info->len = modlen;
+
+	return verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
+				      VERIFY_USE_SECONDARY_KEYRING,
+				      VERIFYING_MODULE_SIGNATURE,
+				      NULL, NULL);
+}
+
+int module_sig_check(struct load_info *info, int flags)
+{
+	int err = -ENODATA;
+	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
+	const char *reason;
+	const void *mod = info->hdr;
+	bool mangled_module = flags & (MODULE_INIT_IGNORE_MODVERSIONS |
+				       MODULE_INIT_IGNORE_VERMAGIC);
+	/*
+	 * Do not allow mangled modules as a module with version information
+	 * removed is no longer the module that was signed.
+	 */
+	if (!mangled_module &&
+	    info->len > markerlen &&
+	    memcmp(mod + info->len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
+		/* We truncate the module to discard the signature */
+		info->len -= markerlen;
+		err = mod_verify_sig(mod, info);
+		if (!err) {
+			info->sig_ok = true;
+			return 0;
+		}
+	}
+
+	/*
+	 * We don't permit modules to be loaded into the trusted kernels
+	 * without a valid signature on them, but if we're not enforcing,
+	 * certain errors are non-fatal.
+	 */
+	switch (err) {
+	case -ENODATA:
+		reason = "unsigned module";
+		break;
+	case -ENOPKG:
+		reason = "module with unsupported crypto";
+		break;
+	case -ENOKEY:
+		reason = "module with unavailable key";
+		break;
+
+	default:
+		/*
+		 * All other errors are fatal, including lack of memory,
+		 * unparseable signatures, and signature check failures --
+		 * even if signatures aren't required.
+		 */
+		return err;
+	}
+
+	if (is_module_sig_enforced()) {
+		pr_notice("Loading of %s is rejected\n", reason);
+		return -EKEYREJECTED;
+	}
+
+	return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
+}
-- 
2.50.1.windows.1