[PATCH 5.17 006/111] random: always wake up entropy writers after extraction

Greg Kroah-Hartman
2022-05-27T08:59:11Z

commit 489c7fc44b5740d377e8cfdbf0851036e493af00 upstream.

Now that POOL_BITS == POOL_MIN_BITS, we must unconditionally wake up
entropy writers after every extraction. Therefore there's no point of
write_wakeup_threshold, so we can move it to the dustbin of unused
compatibility sysctls. While we're at it, we can fix a small comparison
where we were waking up after <= min rather than < min.

Cc: Theodore Ts'o <tytso@mit.edu>
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/admin-guide/sysctl/kernel.rst | 7 ++++-
drivers/char/random.c | 33 +++++++++-------------------
2 files changed, 16 insertions(+), 24 deletions(-)

--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1030,14 +1030,17 @@ This is a directory, with the following
* ``poolsize``: the entropy pool size, in bits;

* ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
- number of seconds between urandom pool reseeding).
+ number of seconds between urandom pool reseeding). This file is
+ writable for compatibility purposes, but writing to it has no effect
+ on any RNG behavior.

* ``uuid``: a UUID generated every time this is retrieved (this can
thus be used to generate UUIDs at will);

* ``write_wakeup_threshold``: when the entropy count drops below this
(as a number of bits), processes waiting to write to ``/dev/random``
- are woken up.
+ are woken up. This file is writable for compatibility purposes, but
+ writing to it has no effect on any RNG behavior.

If ``drivers/char/random.c`` is built with ``ADD_INTERRUPT_BENCH``
defined, these additional entries are present:
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -296,12 +296,6 @@ enum {
*/
static DECLARE_WAIT_QUEUE_HEAD(random_write_wait);
static struct fasync_struct *fasync;
-/*
- * If the entropy count falls under this number of bits, then we
- * should wake up processes which are selecting or polling on write
- * access to /dev/random.
- */
-static int random_write_wakeup_bits = POOL_MIN_BITS;

static DEFINE_SPINLOCK(random_ready_list_lock);
static LIST_HEAD(random_ready_list);
@@ -739,10 +733,8 @@ static void crng_reseed(struct crng_stat
return;
} while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
extract_entropy(buf.key, sizeof(buf.key));
- if (random_write_wakeup_bits) {
- wake_up_interruptible(&random_write_wait);
- kill_fasync(&fasync, SIGIO, POLL_OUT);
- }
+ 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,
@@ -1471,7 +1463,7 @@ static __poll_t random_poll(struct file
mask = 0;
if (crng_ready())
mask |= EPOLLIN | EPOLLRDNORM;
- if (input_pool.entropy_count < random_write_wakeup_bits)
+ if (input_pool.entropy_count < POOL_MIN_BITS)
mask |= EPOLLOUT | EPOLLWRNORM;
return mask;
}
@@ -1556,7 +1548,7 @@ static long random_ioctl(struct file *f,
*/
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
- if (xchg(&input_pool.entropy_count, 0) && random_write_wakeup_bits) {
+ if (xchg(&input_pool.entropy_count, 0)) {
wake_up_interruptible(&random_write_wait);
kill_fasync(&fasync, SIGIO, POLL_OUT);
}
@@ -1636,9 +1628,9 @@ SYSCALL_DEFINE3(getrandom, char __user *

#include <linux/sysctl.h>

-static int min_write_thresh;
-static int max_write_thresh = POOL_BITS;
static int random_min_urandom_seed = 60;
+static int random_write_wakeup_bits = POOL_MIN_BITS;
+static int sysctl_poolsize = POOL_BITS;
static char sysctl_bootid[16];

/*
@@ -1677,7 +1669,6 @@ static int proc_do_uuid(struct ctl_table
return proc_dostring(&fake_table, write, buffer, lenp, ppos);
}

-static int sysctl_poolsize = POOL_BITS;
static struct ctl_table random_table[] = {
{
.procname = "poolsize",
@@ -1698,9 +1689,7 @@ static struct ctl_table random_table[] =
.data = &random_write_wakeup_bits,
.maxlen = sizeof(int),
.mode = 0644,
- .proc_handler = proc_dointvec_minmax,
- .extra1 = &min_write_thresh,
- .extra2 = &max_write_thresh,
+ .proc_handler = proc_dointvec,
},
{
.procname = "urandom_min_reseed_secs",
@@ -1892,13 +1881,13 @@ void add_hwgenerator_randomness(const ch
}

/* Throttle writing if we're above the trickle threshold.
- * We'll be woken up again once below random_write_wakeup_thresh,
- * when the calling thread is about to terminate, or once
- * CRNG_RESEED_INTERVAL has lapsed.
+ * We'll be woken up again once below POOL_MIN_BITS, when
+ * the calling thread is about to terminate, or once
+ * CRNG_RESEED_INTERVAL has elapsed.
*/
wait_event_interruptible_timeout(random_write_wait,
!system_wq || kthread_should_stop() ||
- input_pool.entropy_count <= random_write_wakeup_bits,
+ input_pool.entropy_count < POOL_MIN_BITS,
CRNG_RESEED_INTERVAL);
mix_pool_bytes(buffer, count);
credit_entropy_bits(entropy);