certs: Explain the rationale to call panic()

The blacklist_init() function calls panic() for memory allocation
errors.  This change documents the reason why we don't return -ENODEV.

Link: https://lore.kernel.org/r/20220322111323.542184-2-mic@digikod.net
Link: https://lore.kernel.org/r/YjeW2r6Wv55Du0bJ@iki.fi
Suggested-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

certs/blacklist.c

@@ -307,6 +307,15 @@ static int restrict_link_for_blacklist(struct key *dest_keyring,
 
 /*
  * Initialise the blacklist
+ *
+ * The blacklist_init() function is registered as an initcall via
+ * device_initcall().  As a result if the blacklist_init() function fails for
+ * any reason the kernel continues to execute.  While cleanly returning -ENODEV
+ * could be acceptable for some non-critical kernel parts, if the blacklist
+ * keyring fails to load it defeats the certificate/key based deny list for
+ * signed modules.  If a critical piece of security functionality that users
+ * expect to be present fails to initialize, panic()ing is likely the right
+ * thing to do.
  */
 static int __init blacklist_init(void)
 {