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

Jason A. Donenfeld
2022-02-23T13:13:05Z

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