Re: [PATCH v3 1/2] random: add mechanism for VM forks to reinitialize crng

[Ard Biesheuvel]

On Thu, 24 Feb 2022 at 14:39, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> 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, by force reseeding the crng.
>
> This has the added benefit of also draining the entropy pool and setting
> its timer back, so that any old entropy that was there prior -- which
> could have already been used by a different fork, or generally gone
> stale -- does not contribute to the accounting of the next 256 bits.
>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Jann Horn <jannh@google.com>
> Cc: Eric Biggers <ebiggers@google.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Acked-by: Ard Biesheuvel <ardb@kernel.org>

> ---
>  drivers/char/random.c  | 50 +++++++++++++++++++++++++++++-------------
>  include/linux/random.h |  1 +
>  2 files changed, 36 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 9fb06fc298d3..e8b84791cefe 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -289,14 +289,14 @@ static DEFINE_PER_CPU(struct crng, crngs) = {
>  };
>
>  /* Used by crng_reseed() to extract a new seed from the input pool. */
> -static bool drain_entropy(void *buf, size_t nbytes);
> +static bool drain_entropy(void *buf, size_t nbytes, bool force);
>
>  /*
>   * This extracts a new crng key from the input pool, but only if there is a
> - * sufficient amount of entropy available, in order to mitigate bruteforcing
> - * of newly added bits.
> + * sufficient amount of entropy available or force is true, in order to
> + * mitigate bruteforcing of newly added bits.
>   */
> -static void crng_reseed(void)
> +static void crng_reseed(bool force)
>  {
>         unsigned long flags;
>         unsigned long next_gen;
> @@ -304,7 +304,7 @@ static void crng_reseed(void)
>         bool finalize_init = false;
>
>         /* Only reseed if we can, to prevent brute forcing a small amount of 
> new bits. */
> -       if (!drain_entropy(key, sizeof(key)))
> +       if (!drain_entropy(key, sizeof(key), force))
>                 return;
>
>         /*
> @@ -406,7 +406,7 @@ static void crng_make_state(u32 
> chacha_state[CHACHA_STATE_WORDS],
>          * in turn bumps the generation counter that we check below.
>          */
>         if (unlikely(time_after(jiffies, READ_ONCE(base_crng.birth) + 
> CRNG_RESEED_INTERVAL)))
> -               crng_reseed();
> +               crng_reseed(false);
>
>         local_lock_irqsave(&crngs.lock, flags);
>         crng = raw_cpu_ptr(&crngs);
> @@ -771,10 +771,10 @@ EXPORT_SYMBOL(get_random_bytes_arch);
>   *
>   * Finally, extract entropy via these two, with the latter one
>   * setting the entropy count to zero and extracting only if there
> - * is POOL_MIN_BITS entropy credited prior:
> + * is POOL_MIN_BITS entropy credited prior or force is true:
>   *
>   *     static void extract_entropy(void *buf, size_t nbytes)
> - *     static bool drain_entropy(void *buf, size_t nbytes)
> + *     static bool drain_entropy(void *buf, size_t nbytes, bool force)
>   *
>   **********************************************************************/
>
> @@ -832,7 +832,7 @@ static void credit_entropy_bits(size_t nbits)
>         } while (cmpxchg(&input_pool.entropy_count, orig, entropy_count) != 
> orig);
>
>         if (crng_init < 2 && entropy_count >= POOL_MIN_BITS)
> -               crng_reseed();
> +               crng_reseed(false);
>  }
>
>  /*
> @@ -882,16 +882,16 @@ static void extract_entropy(void *buf, size_t nbytes)
>  }
>
>  /*
> - * First we make sure we have POOL_MIN_BITS of entropy in the pool, and then 
> we
> - * set the entropy count to zero (but don't actually touch any data). Only 
> then
> - * can we extract a new key with extract_entropy().
> + * First we make sure we have POOL_MIN_BITS of entropy in the pool unless 
> force
> + * is true, and then we set the entropy count to zero (but don't actually 
> touch
> + * any data). Only then can we extract a new key with extract_entropy().
>   */
> -static bool drain_entropy(void *buf, size_t nbytes)
> +static bool drain_entropy(void *buf, size_t nbytes, bool force)
>  {
>         unsigned int entropy_count;
>         do {
>                 entropy_count = READ_ONCE(input_pool.entropy_count);
> -               if (entropy_count < POOL_MIN_BITS)
> +               if (!force && entropy_count < POOL_MIN_BITS)
>                         return false;
>         } while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != 
> entropy_count);
>         extract_entropy(buf, nbytes);
> @@ -915,6 +915,7 @@ 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
> @@ -946,6 +947,10 @@ 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 necessarily secret) ID
> + * representing the current instance of a VM to the pool, without crediting,
> + * and then force-reseeds the crng so that it takes effect immediately.
> + *
>   * 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
> @@ -1175,6 +1180,21 @@ 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 immediately force a reseed after so
> + * that it's used by the crng posthaste.
> + */
> +void add_vmfork_randomness(const void *unique_vm_id, size_t size)
> +{
> +       add_device_randomness(unique_vm_id, size);
> +       if (crng_ready()) {
> +               crng_reseed(true);
> +               pr_notice("crng reseeded due to virtual machine fork\n");
> +       }
> +}
> +EXPORT_SYMBOL_GPL(add_vmfork_randomness);
> +
>  struct fast_pool {
>         union {
>                 u32 pool32[4];
> @@ -1564,7 +1584,7 @@ static long random_ioctl(struct file *f, unsigned int 
> cmd, unsigned long arg)
>                         return -EPERM;
>                 if (crng_init < 2)
>                         return -ENODATA;
> -               crng_reseed();
> +               crng_reseed(false);
>                 return 0;
>         default:
>                 return -EINVAL;
> 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
>