linux-kernel - Re: [PATCH] crypto: testmgr - fix RNG performance in fuzz tests

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Date:   Mon, 27 Feb 2023 08:13:41 +0000
From:   "Yann Droneaud" <ydroneaud@...eya.com>
To:     "Eric Biggers" <ebiggers@...nel.org>, linux-crypto@...r.kernel.org,
        "Herbert Xu" <herbert@...dor.apana.org.au>
Cc:     "Jason A . Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] crypto: testmgr - fix RNG performance in fuzz tests

Hi,

27 février 2023 à 06:52 "Eric Biggers" <ebiggers@...nel.org> a écrit:
> 
> From: Eric Biggers <ebiggers@...gle.com>
> 
> The performance of the crypto fuzz tests has greatly regressed since
> v5.18. When booting a kernel on an arm64 dev board with all software
> crypto algorithms and CONFIG_CRYPTO_MANAGER_EXTRA_TESTS enabled, the
> fuzz tests now take about 200 seconds to run, or about 325 seconds with
> lockdep enabled, compared to about 5 seconds before.
> 
> The root cause is that the random number generation has become much
> slower due to commit d4150779e60f ("random32: use real rng for
> non-deterministic randomness"). On my same arm64 dev board, at the time
> the fuzz tests are run, get_random_u8() is about 345x slower than
> prandom_u32_state(), or about 469x if lockdep is enabled.
> 
> Lockdep makes a big difference, but much of the rest comes from the
> get_random_*() functions taking a *very* slow path when the CRNG is not
> yet initialized. Since the crypto self-tests run early during boot,
> even having a hardware RNG driver enabled (CONFIG_CRYPTO_DEV_QCOM_RNG in
> my case) doesn't prevent this. x86 systems don't have this issue, but
> they still see a significant regression if lockdep is enabled.
> 
> Converting the "Fully random bytes" case in generate_random_bytes() to
> use get_random_bytes() helps significantly, improving the test time to
> about 27 seconds. But that's still over 5x slower than before.
> 
> This is all a bit silly, though, since the fuzz tests don't actually
> need cryptographically secure random numbers. So let's just make them
> use a non-cryptographically-secure RNG as they did before. The original
> prandom_u32() is gone now, so let's use prandom_u32_state() instead,
> with an explicitly managed state, like various other self-tests in the
> kernel source tree (rbtree_test.c, test_scanf.c, etc.) already do. This
> also has the benefit that no locking is required anymore, so performance
> should be even better than the original version that used prandom_u32().
> 
> Fixes: d4150779e60f ("random32: use real rng for non-deterministic randomness")
> Cc: stable@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  crypto/testmgr.c | 268 ++++++++++++++++++++++++++++++-----------------
>  1 file changed, 171 insertions(+), 97 deletions(-)
> 
> diff --git a/crypto/testmgr.c b/crypto/testmgr.c
> index c91e93ece20b..2cbd2f8ce3c3 100644
> --- a/crypto/testmgr.c
> +++ b/crypto/testmgr.c
> @@ -860,12 +860,52 @@ static int prepare_keybuf(const u8 *key, unsigned int ksize,
>  
>  #ifdef CONFIG_CRYPTO_MANAGER_EXTRA_TESTS
>  
> +/*
> + * The fuzz tests use prandom instead of the normal Linux RNG since they don't
> + * need cryptographically secure random numbers. This greatly improves the
> + * performance of these tests, especially if they are run before the Linux RNG
> + * has been initialized or if they are run on a lockdep-enabled kernel.
> + */
> +
> +static inline void init_rnd_state(struct rnd_state *rng)
> +{
> + static atomic64_t next_seed;
> +
> + prandom_seed_state(rng, atomic64_inc_return(&next_seed));

Isn't making this deterministic defeating the purpose of fuzzing ?

Regards.

-- 
Yann Droneaud
OPTEYA