random: provide notifier for VM fork

Drivers such as WireGuard need to learn when VMs fork in order to clear
sessions. This commit provides a simple notifier_block for that, with a
register and unregister function. When no VM fork detection is compiled
in, this turns into a no-op, similar to how the power notifier works.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -1140,6 +1140,8 @@ void add_bootloader_randomness(const void *buf, size_t size)
 EXPORT_SYMBOL_GPL(add_bootloader_randomness);
 
 #if IS_ENABLED(CONFIG_VMGENID)
+static BLOCKING_NOTIFIER_HEAD(vmfork_chain);
+
 /*
  * Handle a new unique VM ID, which is unique, not secret, so we
  * don't credit it, but we do immediately force a reseed after so
@@ -1152,10 +1154,23 @@ void add_vmfork_randomness(const void *unique_vm_id, size_t size)
 		crng_reseed(true);
 		pr_notice("crng reseeded due to virtual machine fork\n");
 	}
+	blocking_notifier_call_chain(&vmfork_chain, 0, NULL);
 }
 #if IS_MODULE(CONFIG_VMGENID)
 EXPORT_SYMBOL_GPL(add_vmfork_randomness);
 #endif
+
+int register_random_vmfork_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&vmfork_chain, nb);
+}
+EXPORT_SYMBOL_GPL(register_random_vmfork_notifier);
+
+int unregister_random_vmfork_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&vmfork_chain, nb);
+}
+EXPORT_SYMBOL_GPL(unregister_random_vmfork_notifier);
 #endif
 
 struct fast_pool {

include/linux/random.h

@@ -32,6 +32,11 @@ extern void add_hwgenerator_randomness(const void *buffer, size_t count,
 				       size_t entropy);
 #if IS_ENABLED(CONFIG_VMGENID)
 extern void add_vmfork_randomness(const void *unique_vm_id, size_t size);
+extern int register_random_vmfork_notifier(struct notifier_block *nb);
+extern int unregister_random_vmfork_notifier(struct notifier_block *nb);
+#else
+static inline int register_random_vmfork_notifier(struct notifier_block *nb) { return 0; }
+static inline int unregister_random_vmfork_notifier(struct notifier_block *nb) { return 0; }
 #endif
 
 extern void get_random_bytes(void *buf, size_t nbytes);