[PATCH v3] module: Add log info for verifying module signature

[Yusong Gao <a869920004@gmail.com>]

Add log information in kernel-space when loading module failures.
Try to load the unsigned module and the module with bad signature
when set 1 to /sys/module/module/parameters/sig_enforce.

Unsigned module case:
(linux) insmod unsigned.ko
[   18.714661] Loading of unsigned module is rejected
insmod: can't insert 'unsigned.ko': Key was rejected by service
(linux)

Bad signature module case:
(linux) insmod bad_signature.ko
insmod: can't insert 'bad_signature.ko': Key was rejected by service
(linux)

There have different logging behavior the bad signature case only log
in user-space, add log info for fatal errors in module_sig_check().

Signed-off-by: Yusong Gao <a869920004@gmail.com>
---
V3: Clarify the message type and the error code meaning.
V2: Change print level from notice to debug.
---
 kernel/module/signing.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/module/signing.c b/kernel/module/signing.c
index a2ff4242e623..826cdab8e3e4 100644
--- a/kernel/module/signing.c
+++ b/kernel/module/signing.c
@@ -67,6 +67,31 @@ int mod_verify_sig(const void *mod, struct load_info *info)
 				      NULL, NULL);
 }
 
+static const char *mod_decode_error(int errno)
+{
+	char *errstr = "Unrecognized error";
+
+	switch (errno) {
+	case -ENOMEM:
+		errstr = "Out of memory";
+		break;
+	case -EINVAL:
+		errstr = "Invalid argument";
+		break;
+	case -EBADMSG:
+		errstr = "Invaild module signature format";
+		break;
+	case -EMSGSIZE:
+		errstr = "Message too long";
+		break;
+	case -EKEYREJECTED:
+		errstr = "Key was rejected by service";
+		break;
+	}
+
+	return errstr;
+}
+
 int module_sig_check(struct load_info *info, int flags)
 {
 	int err = -ENODATA;
@@ -113,6 +138,8 @@ int module_sig_check(struct load_info *info, int flags)
 		 * unparseable signatures, and signature check failures --
 		 * even if signatures aren't required.
 		 */
+		pr_debug("Verifying module signature failed: %s\n",
+			 mod_decode_error(err));
 		return err;
 	}
 
-- 
2.34.1