random: Make crng state queryable

It is very useful to be able to know whether or not get_random_bytes_wait
/ wait_for_random_bytes is going to block or not, or whether plain
get_random_bytes is going to return good randomness or bad randomness.

The particular use case is for mitigating certain attacks in WireGuard.
A handshake packet arrives and is queued up. Elsewhere a worker thread
takes items from the queue and processes them. In replying to these
items, it needs to use some random data, and it has to be good random
data. If we simply block until we can have good randomness, then it's
possible for an attacker to fill the queue up with packets waiting to be
processed. Upon realizing the queue is full, WireGuard will detect that
it's under a denial of service attack, and behave accordingly. A better
approach is just to drop incoming handshake packets if the crng is not
yet initialized.

This patch, therefore, makes that information directly accessible.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 561082d46a82d360b72e3d8bebeaff934f2975f5..bf5f99fc36f1b654aad2f4cfe196c9b66814be82 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1663,6 +1663,21 @@ int wait_for_random_bytes(void)
 }
 EXPORT_SYMBOL(wait_for_random_bytes);
 
+/*
+ * Returns whether or not the urandom pool has been seeded and thus guaranteed
+ * to supply cryptographically secure random numbers. This applies to: the
+ * /dev/urandom device, the get_random_bytes function, and the get_random_{u32,
+ * ,u64,int,long} family of functions.
+ *
+ * Returns: true if the urandom pool has been seeded.
+ *          false if the urandom pool has not been seeded.
+ */
+bool rng_is_initialized(void)
+{
+       return crng_ready();
+}
+EXPORT_SYMBOL(rng_is_initialized);
+
 /*
  * Add a callback function that will be invoked when the nonblocking
  * pool is initialised.

diff --git a/include/linux/random.h b/include/linux/random.h
index f1c9bc5cd2315becf0e38abd850bb687bffab768..445a0ea4ff49a1de4edaf8e60ab82e2e36aa5080 100644 (file)
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -36,6 +36,7 @@ extern void add_interrupt_randomness(int irq, int irq_flags) __latent_entropy;
 
 extern void get_random_bytes(void *buf, int nbytes);
 extern int wait_for_random_bytes(void);
+extern bool rng_is_initialized(void);
 extern int add_random_ready_callback(struct random_ready_callback *rdy);
 extern void del_random_ready_callback(struct random_ready_callback *rdy);
 extern int __must_check get_random_bytes_arch(void *buf, int nbytes);