Commit f5b98461 authored by Jason A. Donenfeld's avatar Jason A. Donenfeld Committed by Theodore Ts'o

random: use chacha20 for get_random_int/long

Now that our crng uses chacha20, we can rely on its speedy
characteristics for replacing MD5, while simultaneously achieving a
higher security guarantee. Before the idea was to use these functions if
you wanted random integers that aren't stupidly insecure but aren't
necessarily secure either, a vague gray zone, that hopefully was "good
enough" for its users. With chacha20, we can strengthen this claim,
since either we're using an rdrand-like instruction, or we're using the
same crng as /dev/urandom. And it's faster than what was before.

We could have chosen to replace this with a SipHash-derived function,
which might be slightly faster, but at the cost of having yet another
RNG construction in the kernel. By moving to chacha20, we have a single
RNG to analyze and verify, and we also already get good performance
improvements on all platforms.

Implementation-wise, rather than use a generic buffer for both
get_random_int/long and memcpy based on the size needs, we use a
specific buffer for 32-bit reads and for 64-bit reads. This way, we're
guaranteed to always have aligned accesses on all platforms. While
slightly more verbose in C, the assembly this generates is a lot
simpler than otherwise.

Finally, on 32-bit platforms where longs and ints are the same size,
we simply alias get_random_int to get_random_long.
Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
Suggested-by: default avatarTheodore Ts'o <tytso@mit.edu>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 5d0e5ea3
......@@ -2016,63 +2016,65 @@ struct ctl_table random_table[] = {
};
#endif /* CONFIG_SYSCTL */
static u32 random_int_secret[MD5_MESSAGE_BYTES / 4] ____cacheline_aligned;
int random_int_secret_init(void)
{
get_random_bytes(random_int_secret, sizeof(random_int_secret));
return 0;
}
static DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash)
__aligned(sizeof(unsigned long));
struct batched_entropy {
union {
unsigned long entropy_long[CHACHA20_BLOCK_SIZE / sizeof(unsigned long)];
unsigned int entropy_int[CHACHA20_BLOCK_SIZE / sizeof(unsigned int)];
};
unsigned int position;
};
/*
* Get a random word for internal kernel use only. Similar to urandom but
* with the goal of minimal entropy pool depletion. As a result, the random
* value is not cryptographically secure but for several uses the cost of
* depleting entropy is too high
* Get a random word for internal kernel use only. The quality of the random
* number is either as good as RDRAND or as good as /dev/urandom, with the
* goal of being quite fast and not depleting entropy.
*/
unsigned int get_random_int(void)
static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_long);
unsigned long get_random_long(void)
{
__u32 *hash;
unsigned int ret;
unsigned long ret;
struct batched_entropy *batch;
if (arch_get_random_int(&ret))
if (arch_get_random_long(&ret))
return ret;
hash = get_cpu_var(get_random_int_hash);
hash[0] += current->pid + jiffies + random_get_entropy();
md5_transform(hash, random_int_secret);
ret = hash[0];
put_cpu_var(get_random_int_hash);
batch = &get_cpu_var(batched_entropy_long);
if (batch->position % ARRAY_SIZE(batch->entropy_long) == 0) {
extract_crng((u8 *)batch->entropy_long);
batch->position = 0;
}
ret = batch->entropy_long[batch->position++];
put_cpu_var(batched_entropy_long);
return ret;
}
EXPORT_SYMBOL(get_random_int);
EXPORT_SYMBOL(get_random_long);
/*
* Same as get_random_int(), but returns unsigned long.
*/
unsigned long get_random_long(void)
#if BITS_PER_LONG == 32
unsigned int get_random_int(void)
{
__u32 *hash;
unsigned long ret;
return get_random_long();
}
#else
static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_int);
unsigned int get_random_int(void)
{
unsigned int ret;
struct batched_entropy *batch;
if (arch_get_random_long(&ret))
if (arch_get_random_int(&ret))
return ret;
hash = get_cpu_var(get_random_int_hash);
hash[0] += current->pid + jiffies + random_get_entropy();
md5_transform(hash, random_int_secret);
ret = *(unsigned long *)hash;
put_cpu_var(get_random_int_hash);
batch = &get_cpu_var(batched_entropy_int);
if (batch->position % ARRAY_SIZE(batch->entropy_int) == 0) {
extract_crng((u8 *)batch->entropy_int);
batch->position = 0;
}
ret = batch->entropy_int[batch->position++];
put_cpu_var(batched_entropy_int);
return ret;
}
EXPORT_SYMBOL(get_random_long);
#endif
EXPORT_SYMBOL(get_random_int);
/**
* randomize_page - Generate a random, page aligned address
......
......@@ -37,7 +37,6 @@ extern void get_random_bytes(void *buf, int nbytes);
extern int add_random_ready_callback(struct random_ready_callback *rdy);
extern void del_random_ready_callback(struct random_ready_callback *rdy);
extern void get_random_bytes_arch(void *buf, int nbytes);
extern int random_int_secret_init(void);
#ifndef MODULE
extern const struct file_operations random_fops, urandom_fops;
......
......@@ -879,7 +879,6 @@ static void __init do_basic_setup(void)
do_ctors();
usermodehelper_enable();
do_initcalls();
random_int_secret_init();
}
static void __init do_pre_smp_initcalls(void)
......
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