[PATCH RFC v1 1/2] random: add mechanism for VM forks to reinitialize crng

["Jason A. Donenfeld" <Jason@zx2c4.com>]

When a VM forks, we must immediately mix in additional information to
the stream of random output so that two forks or a rollback don't
produce the same stream of random numbers, which could have catastrophic
cryptographic consequences. This commit adds a simple API, add_vmfork_
randomness(), for that.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c  | 58 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/random.h |  1 +
 2 files changed, 59 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 536237a0f073..29d6ce484d15 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -344,6 +344,46 @@ static void crng_reseed(void)
 	}
 }
 
+/*
+ * This mixes unique_vm_id directly into the base_crng key as soon as
+ * possible, similarly to crng_pre_init_inject(), even if the crng is
+ * already running, in order to immediately branch streams from prior
+ * VM instances.
+ */
+static void crng_vm_fork_inject(const void *unique_vm_id, size_t len)
+{
+	unsigned long flags, next_gen;
+	struct blake2s_state hash;
+
+	/*
+	 * Unlike crng_reseed(), we take the lock as early as possible,
+	 * since we don't want the RNG to be used until it's updated.
+	 */
+	spin_lock_irqsave(&base_crng.lock, flags);
+
+	/*
+	 * Also update the generation, while locked, as early as
+	 * possible. This will mean unlocked reads of the generation
+	 * will cause a reseeding of per-cpu crngs, and those will
+	 * spin on the base_crng lock waiting for the rest of this
+	 * operation to complete, which achieves the goal of blocking
+	 * the production of new output until this is done.
+	 */
+	next_gen = base_crng.generation + 1;
+	if (next_gen == ULONG_MAX)
+		++next_gen;
+	WRITE_ONCE(base_crng.generation, next_gen);
+	WRITE_ONCE(base_crng.birth, jiffies);
+
+	/* This is the same formulation used by crng_pre_init_inject(). */
+	blake2s_init(&hash, sizeof(base_crng.key));
+	blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));
+	blake2s_update(&hash, unique_vm_id, len);
+	blake2s_final(&hash, base_crng.key);
+
+	spin_unlock_irqrestore(&base_crng.lock, flags);
+}
+
 /*
  * This generates a ChaCha block using the provided key, and then
  * immediately overwites that key with half the block. It returns
@@ -935,6 +975,7 @@ static bool drain_entropy(void *buf, size_t nbytes)
  *	void add_hwgenerator_randomness(const void *buffer, size_t count,
  *					size_t entropy);
  *	void add_bootloader_randomness(const void *buf, size_t size);
+ *	void add_vmfork_randomness(const void *unique_vm_id, size_t size);
  *	void add_interrupt_randomness(int irq);
  *
  * add_device_randomness() adds data to the input pool that
@@ -966,6 +1007,11 @@ static bool drain_entropy(void *buf, size_t nbytes)
  * add_device_randomness(), depending on whether or not the configuration
  * option CONFIG_RANDOM_TRUST_BOOTLOADER is set.
  *
+ * add_vmfork_randomness() adds a unique (but not neccessarily secret) ID
+ * representing the current instance of a VM to the pool, without crediting,
+ * and then immediately mixes that ID into the current base_crng key, so
+ * that it takes effect prior to a reseeding.
+ *
  * add_interrupt_randomness() uses the interrupt timing as random
  * inputs to the entropy pool. Using the cycle counters and the irq source
  * as inputs, it feeds the input pool roughly once a second or after 64
@@ -1195,6 +1241,18 @@ void add_bootloader_randomness(const void *buf, size_t size)
 }
 EXPORT_SYMBOL_GPL(add_bootloader_randomness);
 
+/*
+ * Handle a new unique VM ID, which is unique, not secret, so we
+ * don't credit it, but we do mix it into the entropy pool and
+ * inject it into the crng.
+ */
+void add_vmfork_randomness(const void *unique_vm_id, size_t size)
+{
+	add_device_randomness(unique_vm_id, size);
+	crng_vm_fork_inject(unique_vm_id, size);
+}
+EXPORT_SYMBOL_GPL(add_vmfork_randomness);
+
 struct fast_pool {
 	union {
 		u32 pool32[4];
diff --git a/include/linux/random.h b/include/linux/random.h
index 6148b8d1ccf3..51b8ed797732 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -34,6 +34,7 @@ extern void add_input_randomness(unsigned int type, unsigned int code,
 extern void add_interrupt_randomness(int irq) __latent_entropy;
 extern void add_hwgenerator_randomness(const void *buffer, size_t count,
 				       size_t entropy);
+extern void add_vmfork_randomness(const void *unique_vm_id, size_t size);
 
 extern void get_random_bytes(void *buf, size_t nbytes);
 extern int wait_for_random_bytes(void);
-- 
2.35.1