[PATCH v2 1/2] random: add mechanism for VM forks to reinitialize crng

[Jason A. Donenfeld]

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: Dominik Brodowski <linux@dominikbrodowski.net>
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  | 53 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/random.h |  1 +
 2 files changed, 54 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 536237a0f073..95584f6646f9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -508,6 +508,40 @@ static size_t crng_pre_init_inject(const void *input, 
size_t len,
        return len;
 }
 
+/*
+ * 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_vmfork_inject(const void *unique_vm_id, size_t len)
+{
+       unsigned long flags, next_gen;
+       struct blake2s_state hash;
+
+       spin_lock_irqsave(&base_crng.lock, flags);
+
+       /*
+        * 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 function
+        * 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);
+
+       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);
+}
+
 static void _get_random_bytes(void *buf, size_t nbytes)
 {
        u32 chacha_state[CHACHA_STATE_WORDS];
@@ -935,6 +969,7 @@ 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 +1001,11 @@ 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 +1235,19 @@ 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);
+       if (crng_ready())
+               crng_vmfork_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