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random32: make prandom_u32() output unpredictable

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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 f227e3ec3b ("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: f227e3ec3b ("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 ]
Signed-off-by: Willy Tarreau <w@1wt.eu>
This commit is contained in:
George Spelvin 2020-08-09 06:57:44 +00:00 committed by Willy Tarreau
parent f11901ed72
commit c51f8f88d7
4 changed files with 318 additions and 190 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

1
drivers/char/random.c
View file

@ -1277,7 +1277,6 @@ void add_interrupt_randomness(int irq, int irq_flags)
fast_mix(fast_pool);
add_interrupt_bench(cycles);
this_cpu_add(net_rand_state.s1, fast_pool->pool[cycles & 3]);
if (unlikely(crng_init == 0)) {
if ((fast_pool->count >= 64) &&

36
include/linux/prandom.h
View file

@ -16,12 +16,44 @@ void prandom_bytes(void *buf, size_t nbytes);
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);

7
kernel/time/timer.c
View file

@ -1717,13 +1717,6 @@ void update_process_times(int user_tick)
scheduler_tick();
if (IS_ENABLED(CONFIG_POSIX_TIMERS))
run_posix_cpu_timers();
/* 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);
}
/**

464
lib/random32.c
View file

@ -41,16 +41,6 @@
#include <asm/unaligned.h>
#include <trace/events/random.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) __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);