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Subject[PATCH v8 1/7] random: fix crash on multiple early calls to add_bootloader_randomness()
Date
Currently, if CONFIG_RANDOM_TRUST_BOOTLOADER is enabled, multiple calls
to add_bootloader_randomness() are broken and can cause a NULL pointer
dereference, as noted by Ivan T. Ivanov. This is not only a hypothetical
problem, as qemu on arm64 may provide bootloader entropy via EFI and via
devicetree.

On the first call to add_hwgenerator_randomness(), crng_fast_load() is
executed, and if the seed is long enough, crng_init will be set to 1.
On subsequent calls to add_bootloader_randomness() and then to
add_hwgenerator_randomness(), crng_fast_load() will be skipped. Instead,
wait_event_interruptible() and then credit_entropy_bits() will be called.
If the entropy count for that second seed is large enough, that proceeds
to crng_reseed().

However, both wait_event_interruptible() and crng_reseed() depends
(at least in numa_crng_init()) on workqueues. Therefore, test whether
system_wq is already initialized, which is a sufficient indicator that
workqueue_init_early() has progressed far enough.

If we wind up hitting the !system_wq case, we later want to do what
would have been done there when wqs are up, so set a flag, and do that
work later from the rand_initialize() call.

Reported-by: Ivan T. Ivanov <iivanov@suse.de>
Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness")
Cc: stable@vger.kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
[Jason: added crng_need_done state and related logic.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
drivers/char/random.c | 56 +++++++++++++++++++++++++++----------------
1 file changed, 36 insertions(+), 20 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 82db125aaed7..144e8841bff4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -468,6 +468,7 @@ 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;
@@ -835,6 +836,36 @@ static void __init crng_initialize_primary(struct crng_state *crng)
crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
}

+static void crng_finalize_init(struct crng_state *crng)
+{
+ if (crng != &primary_crng || crng_init >= 2)
+ return;
+ 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;
+ 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;
+ }
+}
+
#ifdef CONFIG_NUMA
static void do_numa_crng_init(struct work_struct *work)
{
@@ -989,25 +1020,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
memzero_explicit(&buf, sizeof(buf));
WRITE_ONCE(crng->init_time, jiffies);
spin_unlock_irqrestore(&crng->lock, flags);
- if (crng == &primary_crng && crng_init < 2) {
- invalidate_batched_entropy();
- numa_crng_init();
- 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;
- }
- }
+ crng_finalize_init(crng);
}

static void _extract_crng(struct crng_state *crng,
@@ -1780,6 +1793,8 @@ static void __init init_std_data(struct entropy_store *r)
int __init rand_initialize(void)
{
init_std_data(&input_pool);
+ if (crng_need_final_init)
+ crng_finalize_init(&primary_crng);
crng_initialize_primary(&primary_crng);
crng_global_init_time = jiffies;
if (ratelimit_disable) {
@@ -2288,7 +2303,8 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
* We'll be woken up again once below random_write_wakeup_thresh,
* or when the calling thread is about to terminate.
*/
- wait_event_interruptible(random_write_wait, kthread_should_stop() ||
+ wait_event_interruptible(random_write_wait,
+ !system_wq || kthread_should_stop() ||
ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
mix_pool_bytes(poolp, buffer, count);
credit_entropy_bits(poolp, entropy);
--
2.34.1
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