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Subject[PATCH 4.14 096/237] random: use RDSEED instead of RDRAND in entropy extraction
Date
From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit 28f425e573e906a4c15f8392cc2b1561ef448595 upstream.

When /dev/random was directly connected with entropy extraction, without
any expansion stage, extract_buf() was called for every 10 bytes of data
read from /dev/random. For that reason, RDRAND was used rather than
RDSEED. At the same time, crng_reseed() was still only called every 5
minutes, so there RDSEED made sense.

Those olden days were also a time when the entropy collector did not use
a cryptographic hash function, which meant most bets were off in terms
of real preimage resistance. For that reason too it didn't matter
_that_ much whether RDSEED was mixed in before or after entropy
extraction; both choices were sort of bad.

But now we have a cryptographic hash function at work, and with that we
get real preimage resistance. We also now only call extract_entropy()
every 5 minutes, rather than every 10 bytes. This allows us to do two
important things.

First, we can switch to using RDSEED in extract_entropy(), as Dominik
suggested. Second, we can ensure that RDSEED input always goes into the
cryptographic hash function with other things before being used
directly. This eliminates a category of attacks in which the CPU knows
the current state of the crng and knows that we're going to xor RDSEED
into it, and so it computes a malicious RDSEED. By going through our
hash function, it would require the CPU to compute a preimage on the
fly, which isn't going to happen.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Suggested-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 246 ++++++++++++--------------------------------------
1 file changed, 62 insertions(+), 184 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -323,14 +323,11 @@ static struct crng_state primary_crng =
* its value (from 0->1->2).
*/
static int crng_init = 0;
-static bool crng_need_final_init = false;
#define crng_ready() (likely(crng_init > 1))
static int crng_init_cnt = 0;
-static unsigned long crng_global_init_time = 0;
#define CRNG_INIT_CNT_THRESH (2 * CHACHA20_KEY_SIZE)
-static void _extract_crng(struct crng_state *crng, u8 out[CHACHA20_BLOCK_SIZE]);
-static void _crng_backtrack_protect(struct crng_state *crng,
- u8 tmp[CHACHA20_BLOCK_SIZE], int used);
+static void extract_crng(u8 out[CHACHA20_BLOCK_SIZE]);
+static void crng_backtrack_protect(u8 tmp[CHACHA20_BLOCK_SIZE], int used);
static void process_random_ready_list(void);
static void _get_random_bytes(void *buf, int nbytes);

@@ -365,7 +362,7 @@ static struct {

static void extract_entropy(void *buf, size_t nbytes);

-static void crng_reseed(struct crng_state *crng);
+static void crng_reseed(void);

/*
* This function adds bytes into the entropy "pool". It does not
@@ -464,7 +461,7 @@ static void credit_entropy_bits(int nbit
trace_credit_entropy_bits(nbits, entropy_count, _RET_IP_);

if (crng_init < 2 && entropy_count >= POOL_MIN_BITS)
- crng_reseed(&primary_crng);
+ crng_reseed();
}

/*********************************************************************
@@ -477,14 +474,6 @@ static void credit_entropy_bits(int nbit

static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);

-/*
- * Hack to deal with crazy userspace progams when they are all trying
- * to access /dev/urandom in parallel. The programs are almost
- * certainly doing something terribly wrong, but we'll work around
- * their brain damage.
- */
-static struct crng_state **crng_node_pool __read_mostly;
-
static void invalidate_batched_entropy(void);

static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);
@@ -494,24 +483,6 @@ static int __init parse_trust_cpu(char *
}
early_param("random.trust_cpu", parse_trust_cpu);

-static bool crng_init_try_arch(struct crng_state *crng)
-{
- int i;
- bool arch_init = true;
- unsigned long rv;
-
- for (i = 4; i < 16; i++) {
- if (!arch_get_random_seed_long(&rv) &&
- !arch_get_random_long(&rv)) {
- rv = random_get_entropy();
- arch_init = false;
- }
- crng->state[i] ^= rv;
- }
-
- return arch_init;
-}
-
static bool __init crng_init_try_arch_early(void)
{
int i;
@@ -530,100 +501,17 @@ static bool __init crng_init_try_arch_ea
return arch_init;
}

-static void crng_initialize_secondary(struct crng_state *crng)
-{
- chacha_init_consts(crng->state);
- _get_random_bytes(&crng->state[4], sizeof(u32) * 12);
- crng_init_try_arch(crng);
- crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
-}
-
-static void __init crng_initialize_primary(void)
+static void __init crng_initialize(void)
{
extract_entropy(&primary_crng.state[4], sizeof(u32) * 12);
if (crng_init_try_arch_early() && trust_cpu && crng_init < 2) {
invalidate_batched_entropy();
- numa_crng_init();
crng_init = 2;
pr_notice("crng init done (trusting CPU's manufacturer)\n");
}
primary_crng.init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
}

-static void crng_finalize_init(void)
-{
- if (!system_wq) {
- /* We can't call numa_crng_init until we have workqueues,
- * so mark this for processing later. */
- crng_need_final_init = true;
- return;
- }
-
- invalidate_batched_entropy();
- numa_crng_init();
- crng_init = 2;
- crng_need_final_init = false;
- process_random_ready_list();
- wake_up_interruptible(&crng_init_wait);
- kill_fasync(&fasync, SIGIO, POLL_IN);
- pr_notice("crng init done\n");
- if (unseeded_warning.missed) {
- pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
- unseeded_warning.missed);
- unseeded_warning.missed = 0;
- }
- if (urandom_warning.missed) {
- pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
- urandom_warning.missed);
- urandom_warning.missed = 0;
- }
-}
-
-static void do_numa_crng_init(struct work_struct *work)
-{
- int i;
- struct crng_state *crng;
- struct crng_state **pool;
-
- pool = kcalloc(nr_node_ids, sizeof(*pool), GFP_KERNEL | __GFP_NOFAIL);
- for_each_online_node(i) {
- crng = kmalloc_node(sizeof(struct crng_state),
- GFP_KERNEL | __GFP_NOFAIL, i);
- spin_lock_init(&crng->lock);
- crng_initialize_secondary(crng);
- pool[i] = crng;
- }
- /* pairs with READ_ONCE() in select_crng() */
- if (cmpxchg_release(&crng_node_pool, NULL, pool) != NULL) {
- for_each_node(i)
- kfree(pool[i]);
- kfree(pool);
- }
-}
-
-static DECLARE_WORK(numa_crng_init_work, do_numa_crng_init);
-
-static void numa_crng_init(void)
-{
- if (IS_ENABLED(CONFIG_NUMA))
- schedule_work(&numa_crng_init_work);
-}
-
-static struct crng_state *select_crng(void)
-{
- if (IS_ENABLED(CONFIG_NUMA)) {
- struct crng_state **pool;
- int nid = numa_node_id();
-
- /* pairs with cmpxchg_release() in do_numa_crng_init() */
- pool = READ_ONCE(crng_node_pool);
- if (pool && pool[nid])
- return pool[nid];
- }
-
- return &primary_crng;
-}
-
/*
* crng_fast_load() can be called by code in the interrupt service
* path. So we can't afford to dilly-dally. Returns the number of
@@ -701,73 +589,71 @@ static int crng_slow_load(const u8 *cp,
return 1;
}

-static void crng_reseed(struct crng_state *crng)
+static void crng_reseed(void)
{
unsigned long flags;
- int i;
+ int i, entropy_count;
union {
u8 block[CHACHA20_BLOCK_SIZE];
u32 key[8];
} buf;

- if (crng == &primary_crng) {
- int entropy_count;
- do {
- entropy_count = READ_ONCE(input_pool.entropy_count);
- if (entropy_count < POOL_MIN_BITS)
- return;
- } while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
- extract_entropy(buf.key, sizeof(buf.key));
- wake_up_interruptible(&random_write_wait);
- kill_fasync(&fasync, SIGIO, POLL_OUT);
- } else {
- _extract_crng(&primary_crng, buf.block);
- _crng_backtrack_protect(&primary_crng, buf.block,
- CHACHA20_KEY_SIZE);
- }
- spin_lock_irqsave(&crng->lock, flags);
- for (i = 0; i < 8; i++) {
- unsigned long rv;
- if (!arch_get_random_seed_long(&rv) &&
- !arch_get_random_long(&rv))
- rv = random_get_entropy();
- crng->state[i + 4] ^= buf.key[i] ^ rv;
- }
+ do {
+ entropy_count = READ_ONCE(input_pool.entropy_count);
+ if (entropy_count < POOL_MIN_BITS)
+ return;
+ } while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
+ extract_entropy(buf.key, sizeof(buf.key));
+ wake_up_interruptible(&random_write_wait);
+ kill_fasync(&fasync, SIGIO, POLL_OUT);
+
+ spin_lock_irqsave(&primary_crng.lock, flags);
+ for (i = 0; i < 8; i++)
+ primary_crng.state[i + 4] ^= buf.key[i];
memzero_explicit(&buf, sizeof(buf));
- WRITE_ONCE(crng->init_time, jiffies);
- spin_unlock_irqrestore(&crng->lock, flags);
- if (crng == &primary_crng && crng_init < 2)
- crng_finalize_init();
+ WRITE_ONCE(primary_crng.init_time, jiffies);
+ spin_unlock_irqrestore(&primary_crng.lock, flags);
+ if (crng_init < 2) {
+ invalidate_batched_entropy();
+ crng_init = 2;
+ process_random_ready_list();
+ wake_up_interruptible(&crng_init_wait);
+ kill_fasync(&fasync, SIGIO, POLL_IN);
+ pr_notice("crng init done\n");
+ if (unseeded_warning.missed) {
+ pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
+ unseeded_warning.missed);
+ unseeded_warning.missed = 0;
+ }
+ if (urandom_warning.missed) {
+ pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
+ urandom_warning.missed);
+ urandom_warning.missed = 0;
+ }
+ }
}

-static void _extract_crng(struct crng_state *crng, u8 out[CHACHA20_BLOCK_SIZE])
+static void extract_crng(u8 out[CHACHA20_BLOCK_SIZE])
{
unsigned long flags, init_time;

if (crng_ready()) {
- init_time = READ_ONCE(crng->init_time);
- if (time_after(READ_ONCE(crng_global_init_time), init_time) ||
- time_after(jiffies, init_time + CRNG_RESEED_INTERVAL))
- crng_reseed(crng);
- }
- spin_lock_irqsave(&crng->lock, flags);
- chacha20_block(&crng->state[0], out);
- if (crng->state[12] == 0)
- crng->state[13]++;
- spin_unlock_irqrestore(&crng->lock, flags);
-}
-
-static void extract_crng(u8 out[CHACHA20_BLOCK_SIZE])
-{
- _extract_crng(select_crng(), out);
+ init_time = READ_ONCE(primary_crng.init_time);
+ if (time_after(jiffies, init_time + CRNG_RESEED_INTERVAL))
+ crng_reseed();
+ }
+ spin_lock_irqsave(&primary_crng.lock, flags);
+ chacha20_block(&primary_crng.state[0], out);
+ if (primary_crng.state[12] == 0)
+ primary_crng.state[13]++;
+ spin_unlock_irqrestore(&primary_crng.lock, flags);
}

/*
* Use the leftover bytes from the CRNG block output (if there is
* enough) to mutate the CRNG key to provide backtracking protection.
*/
-static void _crng_backtrack_protect(struct crng_state *crng,
- u8 tmp[CHACHA20_BLOCK_SIZE], int used)
+static void crng_backtrack_protect(u8 tmp[CHACHA20_BLOCK_SIZE], int used)
{
unsigned long flags;
u32 *s, *d;
@@ -778,17 +664,12 @@ static void _crng_backtrack_protect(stru
extract_crng(tmp);
used = 0;
}
- spin_lock_irqsave(&crng->lock, flags);
+ spin_lock_irqsave(&primary_crng.lock, flags);
s = (u32 *)&tmp[used];
- d = &crng->state[4];
+ d = &primary_crng.state[4];
for (i = 0; i < 8; i++)
*d++ ^= *s++;
- spin_unlock_irqrestore(&crng->lock, flags);
-}
-
-static void crng_backtrack_protect(u8 tmp[CHACHA20_BLOCK_SIZE], int used)
-{
- _crng_backtrack_protect(select_crng(), tmp, used);
+ spin_unlock_irqrestore(&primary_crng.lock, flags);
}

static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
@@ -1053,16 +934,17 @@ static void extract_entropy(void *buf, s
unsigned long flags;
u8 seed[BLAKE2S_HASH_SIZE], next_key[BLAKE2S_HASH_SIZE];
struct {
- unsigned long rdrand[32 / sizeof(long)];
+ unsigned long rdseed[32 / sizeof(long)];
size_t counter;
} block;
size_t i;

trace_extract_entropy(nbytes, input_pool.entropy_count);

- for (i = 0; i < ARRAY_SIZE(block.rdrand); ++i) {
- if (!arch_get_random_long(&block.rdrand[i]))
- block.rdrand[i] = random_get_entropy();
+ for (i = 0; i < ARRAY_SIZE(block.rdseed); ++i) {
+ if (!arch_get_random_seed_long(&block.rdseed[i]) &&
+ !arch_get_random_long(&block.rdseed[i]))
+ block.rdseed[i] = random_get_entropy();
}

spin_lock_irqsave(&input_pool.lock, flags);
@@ -1070,7 +952,7 @@ static void extract_entropy(void *buf, s
/* seed = HASHPRF(last_key, entropy_input) */
blake2s_final(&input_pool.hash, seed);

- /* next_key = HASHPRF(seed, RDRAND || 0) */
+ /* next_key = HASHPRF(seed, RDSEED || 0) */
block.counter = 0;
blake2s(next_key, (u8 *)&block, seed, sizeof(next_key), sizeof(block), sizeof(seed));
blake2s_init_key(&input_pool.hash, BLAKE2S_HASH_SIZE, next_key, sizeof(next_key));
@@ -1080,7 +962,7 @@ static void extract_entropy(void *buf, s

while (nbytes) {
i = min_t(size_t, nbytes, BLAKE2S_HASH_SIZE);
- /* output = HASHPRF(seed, RDRAND || ++counter) */
+ /* output = HASHPRF(seed, RDSEED || ++counter) */
++block.counter;
blake2s(buf, (u8 *)&block, seed, i, sizeof(block), sizeof(seed));
nbytes -= i;
@@ -1374,10 +1256,7 @@ static void __init init_std_data(void)
int __init rand_initialize(void)
{
init_std_data();
- if (crng_need_final_init)
- crng_finalize_init();
- crng_initialize_primary();
- crng_global_init_time = jiffies;
+ crng_initialize();
if (ratelimit_disable) {
urandom_warning.interval = 0;
unseeded_warning.interval = 0;
@@ -1547,8 +1426,7 @@ static long random_ioctl(struct file *f,
return -EPERM;
if (crng_init < 2)
return -ENODATA;
- crng_reseed(&primary_crng);
- WRITE_ONCE(crng_global_init_time, jiffies - 1);
+ crng_reseed();
return 0;
default:
return -EINVAL;

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