From c51f8f88d705e06bd696d7510aff22b33eb8e638 Mon Sep 17 00:00:00 2001 From: George Spelvin Date: Sun, 9 Aug 2020 06:57:44 +0000 Subject: random32: make prandom_u32() output unpredictable Non-cryptographic PRNGs may have great statistical properties, but are usually trivially predictable to someone who knows the algorithm, given a small sample of their output. An LFSR like prandom_u32() is particularly simple, even if the sample is widely scattered bits. It turns out the network stack uses prandom_u32() for some things like random port numbers which it would prefer are *not* trivially predictable. Predictability led to a practical DNS spoofing attack. Oops. This patch replaces the LFSR with a homebrew cryptographic PRNG based on the SipHash round function, which is in turn seeded with 128 bits of strong random key. (The authors of SipHash have *not* been consulted about this abuse of their algorithm.) Speed is prioritized over security; attacks are rare, while performance is always wanted. Replacing all callers of prandom_u32() is the quick fix. Whether to reinstate a weaker PRNG for uses which can tolerate it is an open question. Commit f227e3ec3b5c ("random32: update the net random state on interrupt and activity") was an earlier attempt at a solution. This patch replaces it. Reported-by: Amit Klein Cc: Willy Tarreau Cc: Eric Dumazet Cc: "Jason A. Donenfeld" Cc: Andy Lutomirski Cc: Kees Cook Cc: Thomas Gleixner Cc: Peter Zijlstra Cc: Linus Torvalds Cc: tytso@mit.edu Cc: Florian Westphal Cc: Marc Plumb Fixes: f227e3ec3b5c ("random32: update the net random state on interrupt and activity") Signed-off-by: George Spelvin Link: https://lore.kernel.org/netdev/20200808152628.GA27941@SDF.ORG/ [ willy: partial reversal of f227e3ec3b5c; moved SIPROUND definitions to prandom.h for later use; merged George's prandom_seed() proposal; inlined siprand_u32(); replaced the net_rand_state[] array with 4 members to fix a build issue; cosmetic cleanups to make checkpatch happy; fixed RANDOM32_SELFTEST build ] Signed-off-by: Willy Tarreau --- lib/random32.c | 464 +++++++++++++++++++++++++++++++++++---------------------- 1 file changed, 284 insertions(+), 180 deletions(-) (limited to 'lib') diff --git a/lib/random32.c b/lib/random32.c index dfb9981ab798..be9f242a4207 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -41,16 +41,6 @@ #include #include -#ifdef CONFIG_RANDOM32_SELFTEST -static void __init prandom_state_selftest(void); -#else -static inline void prandom_state_selftest(void) -{ -} -#endif - -DEFINE_PER_CPU(struct rnd_state, net_rand_state) __latent_entropy; - /** * prandom_u32_state - seeded pseudo-random number generator. * @state: pointer to state structure holding seeded state. @@ -70,26 +60,6 @@ u32 prandom_u32_state(struct rnd_state *state) } EXPORT_SYMBOL(prandom_u32_state); -/** - * prandom_u32 - pseudo random number generator - * - * A 32 bit pseudo-random number is generated using a fast - * algorithm suitable for simulation. This algorithm is NOT - * considered safe for cryptographic use. - */ -u32 prandom_u32(void) -{ - struct rnd_state *state = &get_cpu_var(net_rand_state); - u32 res; - - res = prandom_u32_state(state); - trace_prandom_u32(res); - put_cpu_var(net_rand_state); - - return res; -} -EXPORT_SYMBOL(prandom_u32); - /** * prandom_bytes_state - get the requested number of pseudo-random bytes * @@ -121,20 +91,6 @@ void prandom_bytes_state(struct rnd_state *state, void *buf, size_t bytes) } EXPORT_SYMBOL(prandom_bytes_state); -/** - * prandom_bytes - get the requested number of pseudo-random bytes - * @buf: where to copy the pseudo-random bytes to - * @bytes: the requested number of bytes - */ -void prandom_bytes(void *buf, size_t bytes) -{ - struct rnd_state *state = &get_cpu_var(net_rand_state); - - prandom_bytes_state(state, buf, bytes); - put_cpu_var(net_rand_state); -} -EXPORT_SYMBOL(prandom_bytes); - static void prandom_warmup(struct rnd_state *state) { /* Calling RNG ten times to satisfy recurrence condition */ @@ -150,96 +106,6 @@ static void prandom_warmup(struct rnd_state *state) prandom_u32_state(state); } -static u32 __extract_hwseed(void) -{ - unsigned int val = 0; - - (void)(arch_get_random_seed_int(&val) || - arch_get_random_int(&val)); - - return val; -} - -static void prandom_seed_early(struct rnd_state *state, u32 seed, - bool mix_with_hwseed) -{ -#define LCG(x) ((x) * 69069U) /* super-duper LCG */ -#define HWSEED() (mix_with_hwseed ? __extract_hwseed() : 0) - state->s1 = __seed(HWSEED() ^ LCG(seed), 2U); - state->s2 = __seed(HWSEED() ^ LCG(state->s1), 8U); - state->s3 = __seed(HWSEED() ^ LCG(state->s2), 16U); - state->s4 = __seed(HWSEED() ^ LCG(state->s3), 128U); -} - -/** - * prandom_seed - add entropy to pseudo random number generator - * @entropy: entropy value - * - * Add some additional entropy to the prandom pool. - */ -void prandom_seed(u32 entropy) -{ - int i; - /* - * No locking on the CPUs, but then somewhat random results are, well, - * expected. - */ - for_each_possible_cpu(i) { - struct rnd_state *state = &per_cpu(net_rand_state, i); - - state->s1 = __seed(state->s1 ^ entropy, 2U); - prandom_warmup(state); - } -} -EXPORT_SYMBOL(prandom_seed); - -/* - * Generate some initially weak seeding values to allow - * to start the prandom_u32() engine. - */ -static int __init prandom_init(void) -{ - int i; - - prandom_state_selftest(); - - for_each_possible_cpu(i) { - struct rnd_state *state = &per_cpu(net_rand_state, i); - u32 weak_seed = (i + jiffies) ^ random_get_entropy(); - - prandom_seed_early(state, weak_seed, true); - prandom_warmup(state); - } - - return 0; -} -core_initcall(prandom_init); - -static void __prandom_timer(struct timer_list *unused); - -static DEFINE_TIMER(seed_timer, __prandom_timer); - -static void __prandom_timer(struct timer_list *unused) -{ - u32 entropy; - unsigned long expires; - - get_random_bytes(&entropy, sizeof(entropy)); - prandom_seed(entropy); - - /* reseed every ~60 seconds, in [40 .. 80) interval with slack */ - expires = 40 + prandom_u32_max(40); - seed_timer.expires = jiffies + msecs_to_jiffies(expires * MSEC_PER_SEC); - - add_timer(&seed_timer); -} - -static void __init __prandom_start_seed_timer(void) -{ - seed_timer.expires = jiffies + msecs_to_jiffies(40 * MSEC_PER_SEC); - add_timer(&seed_timer); -} - void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state) { int i; @@ -259,51 +125,6 @@ void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state) } EXPORT_SYMBOL(prandom_seed_full_state); -/* - * Generate better values after random number generator - * is fully initialized. - */ -static void __prandom_reseed(bool late) -{ - unsigned long flags; - static bool latch = false; - static DEFINE_SPINLOCK(lock); - - /* Asking for random bytes might result in bytes getting - * moved into the nonblocking pool and thus marking it - * as initialized. In this case we would double back into - * this function and attempt to do a late reseed. - * Ignore the pointless attempt to reseed again if we're - * already waiting for bytes when the nonblocking pool - * got initialized. - */ - - /* only allow initial seeding (late == false) once */ - if (!spin_trylock_irqsave(&lock, flags)) - return; - - if (latch && !late) - goto out; - - latch = true; - prandom_seed_full_state(&net_rand_state); -out: - spin_unlock_irqrestore(&lock, flags); -} - -void prandom_reseed_late(void) -{ - __prandom_reseed(true); -} - -static int __init prandom_reseed(void) -{ - __prandom_reseed(false); - __prandom_start_seed_timer(); - return 0; -} -late_initcall(prandom_reseed); - #ifdef CONFIG_RANDOM32_SELFTEST static struct prandom_test1 { u32 seed; @@ -423,7 +244,28 @@ static struct prandom_test2 { { 407983964U, 921U, 728767059U }, }; -static void __init prandom_state_selftest(void) +static u32 __extract_hwseed(void) +{ + unsigned int val = 0; + + (void)(arch_get_random_seed_int(&val) || + arch_get_random_int(&val)); + + return val; +} + +static void prandom_seed_early(struct rnd_state *state, u32 seed, + bool mix_with_hwseed) +{ +#define LCG(x) ((x) * 69069U) /* super-duper LCG */ +#define HWSEED() (mix_with_hwseed ? __extract_hwseed() : 0) + state->s1 = __seed(HWSEED() ^ LCG(seed), 2U); + state->s2 = __seed(HWSEED() ^ LCG(state->s1), 8U); + state->s3 = __seed(HWSEED() ^ LCG(state->s2), 16U); + state->s4 = __seed(HWSEED() ^ LCG(state->s3), 128U); +} + +static int __init prandom_state_selftest(void) { int i, j, errors = 0, runs = 0; bool error = false; @@ -463,5 +305,267 @@ static void __init prandom_state_selftest(void) pr_warn("prandom: %d/%d self tests failed\n", errors, runs); else pr_info("prandom: %d self tests passed\n", runs); + return 0; } +core_initcall(prandom_state_selftest); #endif + +/* + * The prandom_u32() implementation is now completely separate from the + * prandom_state() functions, which are retained (for now) for compatibility. + * + * Because of (ab)use in the networking code for choosing random TCP/UDP port + * numbers, which open DoS possibilities if guessable, we want something + * stronger than a standard PRNG. But the performance requirements of + * the network code do not allow robust crypto for this application. + * + * So this is a homebrew Junior Spaceman implementation, based on the + * lowest-latency trustworthy crypto primitive available, SipHash. + * (The authors of SipHash have not been consulted about this abuse of + * their work.) + * + * Standard SipHash-2-4 uses 2n+4 rounds to hash n words of input to + * one word of output. This abbreviated version uses 2 rounds per word + * of output. + */ + +struct siprand_state { + unsigned long v0; + unsigned long v1; + unsigned long v2; + unsigned long v3; +}; + +static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy; + +/* + * This is the core CPRNG function. As "pseudorandom", this is not used + * for truly valuable things, just intended to be a PITA to guess. + * For maximum speed, we do just two SipHash rounds per word. This is + * the same rate as 4 rounds per 64 bits that SipHash normally uses, + * so hopefully it's reasonably secure. + * + * There are two changes from the official SipHash finalization: + * - We omit some constants XORed with v2 in the SipHash spec as irrelevant; + * they are there only to make the output rounds distinct from the input + * rounds, and this application has no input rounds. + * - Rather than returning v0^v1^v2^v3, return v1+v3. + * If you look at the SipHash round, the last operation on v3 is + * "v3 ^= v0", so "v0 ^ v3" just undoes that, a waste of time. + * Likewise "v1 ^= v2". (The rotate of v2 makes a difference, but + * it still cancels out half of the bits in v2 for no benefit.) + * Second, since the last combining operation was xor, continue the + * pattern of alternating xor/add for a tiny bit of extra non-linearity. + */ +static inline u32 siprand_u32(struct siprand_state *s) +{ + unsigned long v0 = s->v0, v1 = s->v1, v2 = s->v2, v3 = s->v3; + + PRND_SIPROUND(v0, v1, v2, v3); + PRND_SIPROUND(v0, v1, v2, v3); + s->v0 = v0; s->v1 = v1; s->v2 = v2; s->v3 = v3; + return v1 + v3; +} + + +/** + * prandom_u32 - pseudo random number generator + * + * A 32 bit pseudo-random number is generated using a fast + * algorithm suitable for simulation. This algorithm is NOT + * considered safe for cryptographic use. + */ +u32 prandom_u32(void) +{ + struct siprand_state *state = get_cpu_ptr(&net_rand_state); + u32 res = siprand_u32(state); + + trace_prandom_u32(res); + put_cpu_ptr(&net_rand_state); + return res; +} +EXPORT_SYMBOL(prandom_u32); + +/** + * prandom_bytes - get the requested number of pseudo-random bytes + * @buf: where to copy the pseudo-random bytes to + * @bytes: the requested number of bytes + */ +void prandom_bytes(void *buf, size_t bytes) +{ + struct siprand_state *state = get_cpu_ptr(&net_rand_state); + u8 *ptr = buf; + + while (bytes >= sizeof(u32)) { + put_unaligned(siprand_u32(state), (u32 *)ptr); + ptr += sizeof(u32); + bytes -= sizeof(u32); + } + + if (bytes > 0) { + u32 rem = siprand_u32(state); + + do { + *ptr++ = (u8)rem; + rem >>= BITS_PER_BYTE; + } while (--bytes > 0); + } + put_cpu_ptr(&net_rand_state); +} +EXPORT_SYMBOL(prandom_bytes); + +/** + * prandom_seed - add entropy to pseudo random number generator + * @entropy: entropy value + * + * Add some additional seed material to the prandom pool. + * The "entropy" is actually our IP address (the only caller is + * the network code), not for unpredictability, but to ensure that + * different machines are initialized differently. + */ +void prandom_seed(u32 entropy) +{ + int i; + + add_device_randomness(&entropy, sizeof(entropy)); + + for_each_possible_cpu(i) { + struct siprand_state *state = per_cpu_ptr(&net_rand_state, i); + unsigned long v0 = state->v0, v1 = state->v1; + unsigned long v2 = state->v2, v3 = state->v3; + + do { + v3 ^= entropy; + PRND_SIPROUND(v0, v1, v2, v3); + PRND_SIPROUND(v0, v1, v2, v3); + v0 ^= entropy; + } while (unlikely(!v0 || !v1 || !v2 || !v3)); + + WRITE_ONCE(state->v0, v0); + WRITE_ONCE(state->v1, v1); + WRITE_ONCE(state->v2, v2); + WRITE_ONCE(state->v3, v3); + } +} +EXPORT_SYMBOL(prandom_seed); + +/* + * Generate some initially weak seeding values to allow + * the prandom_u32() engine to be started. + */ +static int __init prandom_init_early(void) +{ + int i; + unsigned long v0, v1, v2, v3; + + if (!arch_get_random_long(&v0)) + v0 = jiffies; + if (!arch_get_random_long(&v1)) + v1 = random_get_entropy(); + v2 = v0 ^ PRND_K0; + v3 = v1 ^ PRND_K1; + + for_each_possible_cpu(i) { + struct siprand_state *state; + + v3 ^= i; + PRND_SIPROUND(v0, v1, v2, v3); + PRND_SIPROUND(v0, v1, v2, v3); + v0 ^= i; + + state = per_cpu_ptr(&net_rand_state, i); + state->v0 = v0; state->v1 = v1; + state->v2 = v2; state->v3 = v3; + } + + return 0; +} +core_initcall(prandom_init_early); + + +/* Stronger reseeding when available, and periodically thereafter. */ +static void prandom_reseed(struct timer_list *unused); + +static DEFINE_TIMER(seed_timer, prandom_reseed); + +static void prandom_reseed(struct timer_list *unused) +{ + unsigned long expires; + int i; + + /* + * Reinitialize each CPU's PRNG with 128 bits of key. + * No locking on the CPUs, but then somewhat random results are, + * well, expected. + */ + for_each_possible_cpu(i) { + struct siprand_state *state; + unsigned long v0 = get_random_long(), v2 = v0 ^ PRND_K0; + unsigned long v1 = get_random_long(), v3 = v1 ^ PRND_K1; +#if BITS_PER_LONG == 32 + int j; + + /* + * On 32-bit machines, hash in two extra words to + * approximate 128-bit key length. Not that the hash + * has that much security, but this prevents a trivial + * 64-bit brute force. + */ + for (j = 0; j < 2; j++) { + unsigned long m = get_random_long(); + + v3 ^= m; + PRND_SIPROUND(v0, v1, v2, v3); + PRND_SIPROUND(v0, v1, v2, v3); + v0 ^= m; + } +#endif + /* + * Probably impossible in practice, but there is a + * theoretical risk that a race between this reseeding + * and the target CPU writing its state back could + * create the all-zero SipHash fixed point. + * + * To ensure that never happens, ensure the state + * we write contains no zero words. + */ + state = per_cpu_ptr(&net_rand_state, i); + WRITE_ONCE(state->v0, v0 ? v0 : -1ul); + WRITE_ONCE(state->v1, v1 ? v1 : -1ul); + WRITE_ONCE(state->v2, v2 ? v2 : -1ul); + WRITE_ONCE(state->v3, v3 ? v3 : -1ul); + } + + /* reseed every ~60 seconds, in [40 .. 80) interval with slack */ + expires = round_jiffies(jiffies + 40 * HZ + prandom_u32_max(40 * HZ)); + mod_timer(&seed_timer, expires); +} + +/* + * The random ready callback can be called from almost any interrupt. + * To avoid worrying about whether it's safe to delay that interrupt + * long enough to seed all CPUs, just schedule an immediate timer event. + */ +static void prandom_timer_start(struct random_ready_callback *unused) +{ + mod_timer(&seed_timer, jiffies); +} + +/* + * Start periodic full reseeding as soon as strong + * random numbers are available. + */ +static int __init prandom_init_late(void) +{ + static struct random_ready_callback random_ready = { + .func = prandom_timer_start + }; + int ret = add_random_ready_callback(&random_ready); + + if (ret == -EALREADY) { + prandom_timer_start(&random_ready); + ret = 0; + } + return ret; +} +late_initcall(prandom_init_late); -- cgit v1.2.3 From 3744741adab6d9195551ce30e65e726c7a408421 Mon Sep 17 00:00:00 2001 From: Willy Tarreau Date: Mon, 10 Aug 2020 10:27:42 +0200 Subject: random32: add noise from network and scheduling activity With the removal of the interrupt perturbations in previous random32 change (random32: make prandom_u32() output unpredictable), the PRNG has become 100% deterministic again. While SipHash is expected to be way more robust against brute force than the previous Tausworthe LFSR, there's still the risk that whoever has even one temporary access to the PRNG's internal state is able to predict all subsequent draws till the next reseed (roughly every minute). This may happen through a side channel attack or any data leak. This patch restores the spirit of commit f227e3ec3b5c ("random32: update the net random state on interrupt and activity") in that it will perturb the internal PRNG's statee using externally collected noise, except that it will not pick that noise from the random pool's bits nor upon interrupt, but will rather combine a few elements along the Tx path that are collectively hard to predict, such as dev, skb and txq pointers, packet length and jiffies values. These ones are combined using a single round of SipHash into a single long variable that is mixed with the net_rand_state upon each invocation. The operation was inlined because it produces very small and efficient code, typically 3 xor, 2 add and 2 rol. The performance was measured to be the same (even very slightly better) than before the switch to SipHash; on a 6-core 12-thread Core i7-8700k equipped with a 40G NIC (i40e), the connection rate dropped from 556k/s to 555k/s while the SYN cookie rate grew from 5.38 Mpps to 5.45 Mpps. Link: https://lore.kernel.org/netdev/20200808152628.GA27941@SDF.ORG/ Cc: George Spelvin Cc: Amit Klein Cc: Eric Dumazet Cc: "Jason A. Donenfeld" Cc: Andy Lutomirski Cc: Kees Cook Cc: Thomas Gleixner Cc: Peter Zijlstra Cc: Linus Torvalds Cc: tytso@mit.edu Cc: Florian Westphal Cc: Marc Plumb Tested-by: Sedat Dilek Signed-off-by: Willy Tarreau --- include/linux/prandom.h | 19 +++++++++++++++++++ kernel/time/timer.c | 2 ++ lib/random32.c | 5 +++++ net/core/dev.c | 4 ++++ 4 files changed, 30 insertions(+) (limited to 'lib') diff --git a/include/linux/prandom.h b/include/linux/prandom.h index cc1e71334e53..bbf4b4ad61df 100644 --- a/include/linux/prandom.h +++ b/include/linux/prandom.h @@ -16,6 +16,12 @@ void prandom_bytes(void *buf, size_t nbytes); void prandom_seed(u32 seed); void prandom_reseed_late(void); +DECLARE_PER_CPU(unsigned long, net_rand_noise); + +#define PRANDOM_ADD_NOISE(a, b, c, d) \ + prandom_u32_add_noise((unsigned long)(a), (unsigned long)(b), \ + (unsigned long)(c), (unsigned long)(d)) + #if BITS_PER_LONG == 64 /* * The core SipHash round function. Each line can be executed in @@ -50,6 +56,18 @@ void prandom_reseed_late(void); #error Unsupported BITS_PER_LONG #endif +static inline void prandom_u32_add_noise(unsigned long a, unsigned long b, + unsigned long c, unsigned long d) +{ + /* + * This is not used cryptographically; it's just + * a convenient 4-word hash function. (3 xor, 2 add, 2 rol) + */ + a ^= raw_cpu_read(net_rand_noise); + PRND_SIPROUND(a, b, c, d); + raw_cpu_write(net_rand_noise, d); +} + struct rnd_state { __u32 s1, s2, s3, s4; }; @@ -99,6 +117,7 @@ static inline void prandom_seed_state(struct rnd_state *state, u64 seed) state->s2 = __seed(i, 8U); state->s3 = __seed(i, 16U); state->s4 = __seed(i, 128U); + PRANDOM_ADD_NOISE(state, i, 0, 0); } /* Pseudo random number generator from numerical recipes. */ diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 3e341af741b9..de37e33a868d 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1706,6 +1706,8 @@ void update_process_times(int user_tick) { struct task_struct *p = current; + PRANDOM_ADD_NOISE(jiffies, user_tick, p, 0); + /* Note: this timer irq context must be accounted for as well. */ account_process_tick(p, user_tick); run_local_timers(); diff --git a/lib/random32.c b/lib/random32.c index be9f242a4207..7f047844e494 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -337,6 +337,8 @@ struct siprand_state { }; static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy; +DEFINE_PER_CPU(unsigned long, net_rand_noise); +EXPORT_PER_CPU_SYMBOL(net_rand_noise); /* * This is the core CPRNG function. As "pseudorandom", this is not used @@ -360,9 +362,12 @@ static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy; static inline u32 siprand_u32(struct siprand_state *s) { unsigned long v0 = s->v0, v1 = s->v1, v2 = s->v2, v3 = s->v3; + unsigned long n = raw_cpu_read(net_rand_noise); + v3 ^= n; PRND_SIPROUND(v0, v1, v2, v3); PRND_SIPROUND(v0, v1, v2, v3); + v0 ^= n; s->v0 = v0; s->v1 = v1; s->v2 = v2; s->v3 = v3; return v1 + v3; } diff --git a/net/core/dev.c b/net/core/dev.c index 9499a414d67e..82dc6b48e45f 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -145,6 +145,7 @@ #include #include #include +#include #include "net-sysfs.h" @@ -3558,6 +3559,7 @@ static int xmit_one(struct sk_buff *skb, struct net_device *dev, dev_queue_xmit_nit(skb, dev); len = skb->len; + PRANDOM_ADD_NOISE(skb, dev, txq, len + jiffies); trace_net_dev_start_xmit(skb, dev); rc = netdev_start_xmit(skb, dev, txq, more); trace_net_dev_xmit(skb, rc, dev, len); @@ -4130,6 +4132,7 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev) if (!skb) goto out; + PRANDOM_ADD_NOISE(skb, dev, txq, jiffies); HARD_TX_LOCK(dev, txq, cpu); if (!netif_xmit_stopped(txq)) { @@ -4195,6 +4198,7 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) skb_set_queue_mapping(skb, queue_id); txq = skb_get_tx_queue(dev, skb); + PRANDOM_ADD_NOISE(skb, dev, txq, jiffies); local_bh_disable(); -- cgit v1.2.3 From c6e169bc146a76d5ccbf4d3825f705414352bd03 Mon Sep 17 00:00:00 2001 From: Willy Tarreau Date: Sat, 24 Oct 2020 18:36:27 +0200 Subject: random32: add a selftest for the prandom32 code Given that this code is new, let's add a selftest for it as well. It doesn't rely on fixed sets, instead it picks 1024 numbers and verifies that they're not more correlated than desired. Link: https://lore.kernel.org/netdev/20200808152628.GA27941@SDF.ORG/ Cc: George Spelvin Cc: Amit Klein Cc: Eric Dumazet Cc: "Jason A. Donenfeld" Cc: Andy Lutomirski Cc: Kees Cook Cc: Thomas Gleixner Cc: Peter Zijlstra Cc: Linus Torvalds Cc: tytso@mit.edu Cc: Florian Westphal Cc: Marc Plumb Signed-off-by: Willy Tarreau --- lib/random32.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) (limited to 'lib') diff --git a/lib/random32.c b/lib/random32.c index 7f047844e494..4d0e05e471d7 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -38,6 +38,7 @@ #include #include #include +#include #include #include @@ -556,6 +557,61 @@ static void prandom_timer_start(struct random_ready_callback *unused) mod_timer(&seed_timer, jiffies); } +#ifdef CONFIG_RANDOM32_SELFTEST +/* Principle: True 32-bit random numbers will all have 16 differing bits on + * average. For each 32-bit number, there are 601M numbers differing by 16 + * bits, and 89% of the numbers differ by at least 12 bits. Note that more + * than 16 differing bits also implies a correlation with inverted bits. Thus + * we take 1024 random numbers and compare each of them to the other ones, + * counting the deviation of correlated bits to 16. Constants report 32, + * counters 32-log2(TEST_SIZE), and pure randoms, around 6 or lower. With the + * u32 total, TEST_SIZE may be as large as 4096 samples. + */ +#define TEST_SIZE 1024 +static int __init prandom32_state_selftest(void) +{ + unsigned int x, y, bits, samples; + u32 xor, flip; + u32 total; + u32 *data; + + data = kmalloc(sizeof(*data) * TEST_SIZE, GFP_KERNEL); + if (!data) + return 0; + + for (samples = 0; samples < TEST_SIZE; samples++) + data[samples] = prandom_u32(); + + flip = total = 0; + for (x = 0; x < samples; x++) { + for (y = 0; y < samples; y++) { + if (x == y) + continue; + xor = data[x] ^ data[y]; + flip |= xor; + bits = hweight32(xor); + total += (bits - 16) * (bits - 16); + } + } + + /* We'll return the average deviation as 2*sqrt(corr/samples), which + * is also sqrt(4*corr/samples) which provides a better resolution. + */ + bits = int_sqrt(total / (samples * (samples - 1)) * 4); + if (bits > 6) + pr_warn("prandom32: self test failed (at least %u bits" + " correlated, fixed_mask=%#x fixed_value=%#x\n", + bits, ~flip, data[0] & ~flip); + else + pr_info("prandom32: self test passed (less than %u bits" + " correlated)\n", + bits+1); + kfree(data); + return 0; +} +core_initcall(prandom32_state_selftest); +#endif /* CONFIG_RANDOM32_SELFTEST */ + /* * Start periodic full reseeding as soon as strong * random numbers are available. -- cgit v1.2.3