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random: warn when kernel uses unseeded randomness

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This enables an important dmesg notification about when drivers have
used the crng without it being seeded first. Prior, these errors would
occur silently, and so there hasn't been a great way of diagnosing these
types of bugs for obscure setups. By adding this as a config option, we
can leave it on by default, so that we learn where these issues happen,
in the field, will still allowing some people to turn it off, if they
really know what they're doing and do not want the log entries.

However, we don't leave it _completely_ by default. An earlier version
of this patch simply had `default y`. I'd really love that, but it turns
out, this problem with unseeded randomness being used is really quite
present and is going to take a long time to fix. Thus, as a compromise
between log-messages-for-all and nobody-knows, this is `default y`,
except it is also `depends on DEBUG_KERNEL`. This will ensure that the
curious see the messages while others don't have to.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Jason A. Donenfeld 2017-06-07 23:06:55 -04:00 committed by Theodore Ts'o
parent 7aed9f72c3
commit d06bfd1989
2 changed files with 29 additions and 2 deletions
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15
drivers/char/random.c
View File

@ -288,7 +288,6 @@
#define SEC_XFER_SIZE 512
#define EXTRACT_SIZE 10
#define DEBUG_RANDOM_BOOT 0
#define LONGS(x) (((x) + sizeof(unsigned long) - 1)/sizeof(unsigned long))
@ -1481,7 +1480,7 @@ void get_random_bytes(void *buf, int nbytes)
{
__u8 tmp[CHACHA20_BLOCK_SIZE];
#if DEBUG_RANDOM_BOOT > 0
#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);
@ -2075,6 +2074,12 @@ 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
batch = &get_cpu_var(batched_entropy_u64);
if (use_lock)
read_lock_irqsave(&batched_entropy_reset_lock, flags);
@ -2101,6 +2106,12 @@ u32 get_random_u32(void)
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
batch = &get_cpu_var(batched_entropy_u32);
if (use_lock)
read_lock_irqsave(&batched_entropy_reset_lock, flags);

16
lib/Kconfig.debug
View File

@ -1209,6 +1209,22 @@ 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
help
Some parts of the kernel contain bugs relating to their use of
cryptographically secure random numbers before it's actually possible
to generate those numbers securely. This setting ensures that these
flaws don't go unnoticed, by enabling a message, should this ever
occur. This will allow people with obscure setups to know when things
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.
config DEBUG_KOBJECT
bool "kobject debugging"
depends on DEBUG_KERNEL