[v3 PATCH] hwrng: core - Remove add_early_randomness

Herbert Xu
2024-05-22T05:37:59Z

On Tue, May 21, 2024 at 03:37:16PM -0400, Nícolas F. R. A. Prado wrote:
>
> FWIW this patch fixes the warning. So feel free to add
>
> Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

Could you please test this patch instead?

---8<---
A potential deadlock was reported with the config file at

https://web.archive.org/web/20240522052129/https://0x0.st/XPN_.txt

In this particular configuration, the deadlock doesn't exist because
the warning triggered at a point before modules were even available.
However, the deadlock can be real because any module loaded would
invoke async_synchronize_full.

The issue is spurious for software crypto algorithms which aren't
themselves involved in async probing. However, it would be hard to
avoid for a PCI crypto driver using async probing.

In this particular call trace, the problem is easily avoided because
the only reason the module is being requested during probing is the
add_early_randomness call in the hwrng core. This feature is
vestigial since there is now a kernel thread dedicated to doing
exactly this.

So remove add_early_randomness as it is no longer needed.

Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
Link: https://lore.kernel.org/r/119dc5ed-f159-41be-9dda-1a056f29888d@notapiano/
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index f5c71a617a99..4084df65c9fa 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -64,19 +64,6 @@ static size_t rng_buffer_size(void)
return RNG_BUFFER_SIZE;
}

-static void add_early_randomness(struct hwrng *rng)
-{
- int bytes_read;
-
- mutex_lock(&reading_mutex);
- bytes_read = rng_get_data(rng, rng_fillbuf, 32, 0);
- mutex_unlock(&reading_mutex);
- if (bytes_read > 0) {
- size_t entropy = bytes_read * 8 * rng->quality / 1024;
- add_hwgenerator_randomness(rng_fillbuf, bytes_read, entropy, false);
- }
-}
-
static inline void cleanup_rng(struct kref *kref)
{
struct hwrng *rng = container_of(kref, struct hwrng, ref);
@@ -340,13 +327,12 @@ static ssize_t rng_current_store(struct device *dev,
const char *buf, size_t len)
{
int err;
- struct hwrng *rng, *old_rng, *new_rng;
+ struct hwrng *rng, *new_rng;

err = mutex_lock_interruptible(&rng_mutex);
if (err)
return -ERESTARTSYS;

- old_rng = current_rng;
if (sysfs_streq(buf, "")) {
err = enable_best_rng();
} else {
@@ -362,11 +348,8 @@ static ssize_t rng_current_store(struct device *dev,
new_rng = get_current_rng_nolock();
mutex_unlock(&rng_mutex);

- if (new_rng) {
- if (new_rng != old_rng)
- add_early_randomness(new_rng);
+ if (new_rng)
put_rng(new_rng);
- }

return err ? : len;
}
@@ -544,7 +527,6 @@ int hwrng_register(struct hwrng *rng)
{
int err = -EINVAL;
struct hwrng *tmp;
- bool is_new_current = false;

if (!rng->name || (!rng->data_read && !rng->read))
goto out;
@@ -573,25 +555,8 @@ int hwrng_register(struct hwrng *rng)
err = set_current_rng(rng);
if (err)
goto out_unlock;
- /* to use current_rng in add_early_randomness() we need
- * to take a ref
- */
- is_new_current = true;
- kref_get(&rng->ref);
}
mutex_unlock(&rng_mutex);
- if (is_new_current || !rng->init) {
- /*
- * Use a new device's input to add some randomness to
- * the system. If this rng device isn't going to be
- * used right away, its init function hasn't been
- * called yet by set_current_rng(); so only use the
- * randomness from devices that don't need an init callback
- */
- add_early_randomness(rng);
- }
- if (is_new_current)
- put_rng(rng);
return 0;
out_unlock:
mutex_unlock(&rng_mutex);
@@ -602,12 +567,11 @@ EXPORT_SYMBOL_GPL(hwrng_register);

void hwrng_unregister(struct hwrng *rng)
{
- struct hwrng *old_rng, *new_rng;
+ struct hwrng *new_rng;
int err;

mutex_lock(&rng_mutex);

- old_rng = current_rng;
list_del(&rng->list);
complete_all(&rng->dying);
if (current_rng == rng) {
@@ -626,11 +590,8 @@ void hwrng_unregister(struct hwrng *rng)
} else
mutex_unlock(&rng_mutex);

- if (new_rng) {
- if (old_rng != new_rng)
- add_early_randomness(new_rng);
+ if (new_rng)
put_rng(new_rng);
- }

wait_for_completion(&rng->cleanup_done);
}
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt