[PATCH 2/3] random: provide notifier for VM fork

[Jason A. Donenfeld]

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@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 drivers/char/random.c  | 15 +++++++++++++++
 include/linux/random.h |  5 +++++
 2 files changed, 20 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6bd1bbab7392..483fd2dc2057 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1141,6 +1141,8 @@ 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_notifier);
+
 /*
  * 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
@@ -1152,11 +1154,24 @@ void add_vmfork_randomness(const void *unique_vm_id, size_t size)
 	if (crng_ready()) {
 		crng_reseed(true);
 		pr_notice("crng reseeded due to virtual machine fork\n");
+		blocking_notifier_call_chain(&vmfork_notifier, 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_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(register_random_vmfork_notifier);
+
+int unregister_random_vmfork_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&vmfork_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(unregister_random_vmfork_notifier);
 #endif
 
 struct fast_pool {
diff --git a/include/linux/random.h b/include/linux/random.h
index e84b6fa27435..7fccbc7e5a75 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -31,6 +31,11 @@ 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);
-- 
2.35.1