[PATCH 4.4 46/64] random32: make prandom_u32() output unpredictable

Greg Kroah-Hartman
2020-11-17T14:07:17Z

From: George Spelvin <lkml@sdf.org>

commit c51f8f88d705e06bd696d7510aff22b33eb8e638 upstream.

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 <aksecurity@gmail.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: tytso@mit.edu
Cc: Florian Westphal <fw@strlen.de>
Cc: Marc Plumb <lkml.mplumb@gmail.com>
Fixes: f227e3ec3b5c ("random32: update the net random state on interrupt and activity")
Signed-off-by: George Spelvin <lkml@sdf.org>
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 ]
[wt: backported to 4.4 -- no latent_entropy, drop prandom_reseed_late]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 2
include/linux/prandom.h | 36 +++
kernel/time/timer.c | 7
lib/random32.c | 463 +++++++++++++++++++++++++++++-------------------
4 files changed, 317 insertions(+), 191 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -678,7 +678,6 @@ retry:
r->initialized = 1;
r->entropy_total = 0;
if (r == &nonblocking_pool) {
- prandom_reseed_late();
process_random_ready_list();
wake_up_all(&urandom_init_wait);
pr_notice("random: %s pool is initialized\n", r->name);
@@ -923,7 +922,6 @@ void add_interrupt_randomness(int irq, i

fast_mix(fast_pool);
add_interrupt_bench(cycles);
- this_cpu_add(net_rand_state.s1, fast_pool->pool[cycles & 3]);

if ((fast_pool->count < 64) &&
!time_after(now, fast_pool->last + HZ))
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -16,12 +16,44 @@ void prandom_bytes(void *buf, size_t nby
void prandom_seed(u32 seed);
void prandom_reseed_late(void);

+#if BITS_PER_LONG == 64
+/*
+ * The core SipHash round function. Each line can be executed in
+ * parallel given enough CPU resources.
+ */
+#define PRND_SIPROUND(v0, v1, v2, v3) ( \
+ v0 += v1, v1 = rol64(v1, 13), v2 += v3, v3 = rol64(v3, 16), \
+ v1 ^= v0, v0 = rol64(v0, 32), v3 ^= v2, \
+ v0 += v3, v3 = rol64(v3, 21), v2 += v1, v1 = rol64(v1, 17), \
+ v3 ^= v0, v1 ^= v2, v2 = rol64(v2, 32) \
+)
+
+#define PRND_K0 (0x736f6d6570736575 ^ 0x6c7967656e657261)
+#define PRND_K1 (0x646f72616e646f6d ^ 0x7465646279746573)
+
+#elif BITS_PER_LONG == 32
+/*
+ * On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
+ * This is weaker, but 32-bit machines are not used for high-traffic
+ * applications, so there is less output for an attacker to analyze.
+ */
+#define PRND_SIPROUND(v0, v1, v2, v3) ( \
+ v0 += v1, v1 = rol32(v1, 5), v2 += v3, v3 = rol32(v3, 8), \
+ v1 ^= v0, v0 = rol32(v0, 16), v3 ^= v2, \
+ v0 += v3, v3 = rol32(v3, 7), v2 += v1, v1 = rol32(v1, 13), \
+ v3 ^= v0, v1 ^= v2, v2 = rol32(v2, 16) \
+)
+#define PRND_K0 0x6c796765
+#define PRND_K1 0x74656462
+
+#else
+#error Unsupported BITS_PER_LONG
+#endif
+
struct rnd_state {
__u32 s1, s2, s3, s4;
};

-DECLARE_PER_CPU(struct rnd_state, net_rand_state);
-
u32 prandom_u32_state(struct rnd_state *state);
void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1432,13 +1432,6 @@ void update_process_times(int user_tick)
#endif
scheduler_tick();
run_posix_cpu_timers(p);
-
- /* The current CPU might make use of net randoms without receiving IRQs
- * to renew them often enough. Let's update the net_rand_state from a
- * non-constant value that's not affine to the number of calls to make
- * sure it's updated when there's some activity (we don't care in idle).
- */
- this_cpu_add(net_rand_state.s1, rol32(jiffies, 24) + user_tick);
}

/*
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -39,16 +39,6 @@
#include <linux/sched.h>
#include <asm/unaligned.h>

-#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);
-
/**
* prandom_u32_state - seeded pseudo-random number generator.
* @state: pointer to state structure holding seeded state.
@@ -69,25 +59,6 @@ u32 prandom_u32_state(struct rnd_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);
- put_cpu_var(state);
-
- return res;
-}
-EXPORT_SYMBOL(prandom_u32);
-
-/**
* prandom_bytes_state - get the requested number of pseudo-random bytes
*
* @state: pointer to state structure holding seeded state.
@@ -118,20 +89,6 @@ void prandom_bytes_state(struct rnd_stat
}
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(state);
-}
-EXPORT_SYMBOL(prandom_bytes);
-
static void prandom_warmup(struct rnd_state *state)
{
/* Calling RNG ten times to satisfy recurrence condition */
@@ -147,97 +104,6 @@ static void prandom_warmup(struct rnd_st
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
- * @seed: seed value
- *
- * Add some additional seeding 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(unsigned long dontcare);
-
-static DEFINE_TIMER(seed_timer, __prandom_timer, 0, 0);
-
-static void __prandom_timer(unsigned long dontcare)
-{
- 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)
-{
- set_timer_slack(&seed_timer, HZ);
- 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;
@@ -256,51 +122,6 @@ void prandom_seed_full_state(struct rnd_
}
}

-/*
- * 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;
@@ -420,7 +241,28 @@ 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;
@@ -460,5 +302,266 @@ static void __init prandom_state_selftes
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);
+
+/*
+ * 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);
+
+ 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(unsigned long dontcare);
+
+static DEFINE_TIMER(seed_timer, prandom_reseed, 0, 0);
+
+static void prandom_reseed(unsigned long dontcare)
+{
+ 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);