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Commit eecabf56 authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

random: suppress spammy warnings about unseeded randomness 



Unfortunately, on some models of some architectures getting a fully
seeded CRNG is extremely difficult, and so this can result in dmesg
getting spammed for a surprisingly long time.  This is really bad from
a security perspective, and so architecture maintainers really need to
do what they can to get the CRNG seeded sooner after the system is
booted.  However, users can't do anything actionble to address this,
and spamming the kernel messages log will only just annoy people.

For developers who want to work on improving this situation,
CONFIG_WARN_UNSEEDED_RANDOM has been renamed to
CONFIG_WARN_ALL_UNSEEDED_RANDOM.  By default the kernel will always
print the first use of unseeded randomness.  This way, hopefully the
security obsessed will be happy that there is _some_ indication when
the kernel boots there may be a potential issue with that architecture
or subarchitecture.  To see all uses of unseeded randomness,
developers can enable CONFIG_WARN_ALL_UNSEEDED_RANDOM.

Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent d06bfd19

drivers/char/random.c

+39 −17
Original line number Diff line number Diff line
@@ -436,6 +436,7 @@ static void _extract_crng(struct crng_state *crng, 
static void _crng_backtrack_protect(struct crng_state *crng,

				    __u8 tmp[CHACHA20_BLOCK_SIZE], int used);

static void process_random_ready_list(void);

static void _get_random_bytes(void *buf, int nbytes);



/**********************************************************************

 *
@@ -776,7 +777,7 @@ static void crng_initialize(struct crng_state *crng) 
		_extract_entropy(&input_pool, &crng->state[4],

				 sizeof(__u32) * 12, 0);

	else

		get_random_bytes(&crng->state[4], sizeof(__u32) * 12);

		_get_random_bytes(&crng->state[4], sizeof(__u32) * 12);

	for (i = 4; i < 16; i++) {

		if (!arch_get_random_seed_long(&rv) &&

		    !arch_get_random_long(&rv))
@@ -1466,6 +1467,30 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf, 
	return ret;

}



#define warn_unseeded_randomness(previous) \

	_warn_unseeded_randomness(__func__, (void *) _RET_IP_, (previous))



static void _warn_unseeded_randomness(const char *func_name, void *caller,

				      void **previous)

{

#ifdef CONFIG_WARN_ALL_UNSEEDED_RANDOM

	const bool print_once = false;

#else

	static bool print_once __read_mostly;

#endif



	if (print_once ||

	    crng_ready() ||

	    (previous && (caller == READ_ONCE(*previous))))

		return;

	WRITE_ONCE(*previous, caller);

#ifndef CONFIG_WARN_ALL_UNSEEDED_RANDOM

	print_once = true;

#endif

	pr_notice("random: %s called from %pF with crng_init=%d\n",

		  func_name, caller, crng_init);

}



/*

 * This function is the exported kernel interface.  It returns some

 * number of good random numbers, suitable for key generation, seeding
@@ -1476,15 +1501,10 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf, 
 * wait_for_random_bytes() should be called and return 0 at least once

 * at any point prior.

 */

void get_random_bytes(void *buf, int nbytes)

static void _get_random_bytes(void *buf, int nbytes)

{

	__u8 tmp[CHACHA20_BLOCK_SIZE];



#ifdef CONFIG_WARN_UNSEEDED_RANDOM

	if (!crng_ready())

		printk(KERN_NOTICE "random: %pF get_random_bytes called "

		       "with crng_init = %d\n", (void *) _RET_IP_, crng_init);

#endif

	trace_get_random_bytes(nbytes, _RET_IP_);



	while (nbytes >= CHACHA20_BLOCK_SIZE) {
@@ -1501,6 +1521,14 @@ void get_random_bytes(void *buf, int nbytes) 
		crng_backtrack_protect(tmp, CHACHA20_BLOCK_SIZE);

	memzero_explicit(tmp, sizeof(tmp));

}



void get_random_bytes(void *buf, int nbytes)

{

	static void *previous;



	warn_unseeded_randomness(&previous);

	_get_random_bytes(buf, nbytes);

}

EXPORT_SYMBOL(get_random_bytes);



/*
@@ -2064,6 +2092,7 @@ u64 get_random_u64(void) 
	bool use_lock = READ_ONCE(crng_init) < 2;

	unsigned long flags = 0;

	struct batched_entropy *batch;

	static void *previous;



#if BITS_PER_LONG == 64

	if (arch_get_random_long((unsigned long *)&ret))
@@ -2074,11 +2103,7 @@ u64 get_random_u64(void) 
	    return ret;

#endif



#ifdef CONFIG_WARN_UNSEEDED_RANDOM

	if (!crng_ready())

		printk(KERN_NOTICE "random: %pF get_random_u64 called "

		       "with crng_init = %d\n", (void *) _RET_IP_, crng_init);

#endif

	warn_unseeded_randomness(&previous);



	batch = &get_cpu_var(batched_entropy_u64);

	if (use_lock)
@@ -2102,15 +2127,12 @@ u32 get_random_u32(void) 
	bool use_lock = READ_ONCE(crng_init) < 2;

	unsigned long flags = 0;

	struct batched_entropy *batch;

	static void *previous;



	if (arch_get_random_int(&ret))

		return ret;



#ifdef CONFIG_WARN_UNSEEDED_RANDOM

	if (!crng_ready())

		printk(KERN_NOTICE "random: %pF get_random_u32 called "

		       "with crng_init = %d\n", (void *) _RET_IP_, crng_init);

#endif

	warn_unseeded_randomness(&previous);



	batch = &get_cpu_var(batched_entropy_u32);

	if (use_lock)

lib/Kconfig.debug

+18 −6
Original line number Diff line number Diff line
@@ -1209,10 +1209,9 @@ config STACKTRACE 
	  It is also used by various kernel debugging features that require

	  stack trace generation.



config WARN_UNSEEDED_RANDOM

	bool "Warn when kernel uses unseeded randomness"

	default y

	depends on DEBUG_KERNEL

config WARN_ALL_UNSEEDED_RANDOM

	bool "Warn for all uses of unseeded randomness"

	default n

	help

	  Some parts of the kernel contain bugs relating to their use of

	  cryptographically secure random numbers before it's actually possible
@@ -1222,8 +1221,21 @@ config WARN_UNSEEDED_RANDOM 
	  are going wrong, so that they might contact developers about fixing

	  it.



	  Say Y here, unless you simply do not care about using unseeded

	  randomness and do not want a potential warning message in your logs.

	  Unfortunately, on some models of some architectures getting

	  a fully seeded CRNG is extremely difficult, and so this can

	  result in dmesg getting spammed for a surprisingly long

	  time.  This is really bad from a security perspective, and

	  so architecture maintainers really need to do what they can

	  to get the CRNG seeded sooner after the system is booted.

	  However, since users can not do anything actionble to

	  address this, by default the kernel will issue only a single

	  warning for the first use of unseeded randomness.



	  Say Y here if you want to receive warnings for all uses of

	  unseeded randomness.  This will be of use primarily for

	  those developers interersted in improving the security of

	  Linux kernels running on their architecture (or

	  subarchitecture).



config DEBUG_KOBJECT

	bool "kobject debugging"