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Subject[RFC PATCH 16/41] random: convert random_ioctl() to queued_entropy API
Date
In an effort to drop credit_entropy_bits_safe() in favor of the new
queue_entropy()/dispatch_queued_entropy() API, convert random_ioctl() from
the former to the latter.

Implement two helpers:
- queue_entropy_bits_safe(), which checks the entropy passed from userspace
for extreme values in analogy to what credit_entropy_bits_safe() did
- discard_queue_entropy(), which is invoked from random_ioctly() to discard
the entropy queued prior to the write_pool() call in case the latter
fails.

Use them to convert the two call sites of credit_entropy_bits_safe()
in random_ioctl() to the new API.

As a side effect, the pool entropy watermark as tracked over the duration
of the write_pool() operation is now taken correctly taken into account
when calulating the amount of new entropy to dispatch to the pool based on
the latter's fill level.

Signed-off-by: Nicolai Stange <nstange@suse.de>
---
drivers/char/random.c | 57 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 53 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 78e65367ea86..03eadefabbca 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -737,7 +737,9 @@ struct queued_entropy {
* dispatch. However, any such sequence of invocations must eventually
* be followed by exactly one call to either of __dequeue_entropy(),
* __dispatch_queued_entropy_fast() or dispatch_queued_entropy()
- * when the actual pool mixing has completed.
+ * when the actual pool mixing has completed. Alternatively,
+ * discard_queued_entropy() may be called in case the mixing has
+ * failed.
* __queue_entropy() must be called with r->lock held.
*
* Entropy extraction is a two-step process:
@@ -813,6 +815,26 @@ static void queue_entropy(struct entropy_store *r, struct queued_entropy *q,
spin_unlock_irqrestore(&r->lock, flags);
}

+/*
+ * Queue entropy which comes from userspace and might take extreme
+ * values.
+ */
+static int queue_entropy_bits_safe(struct entropy_store *r,
+ struct queued_entropy *q,
+ int nbits)
+{
+ const int nbits_max = r->poolinfo->poolwords * 32;
+
+ if (nbits < 0)
+ return -EINVAL;
+
+ /* Cap the value to avoid overflows */
+ nbits = min(nbits, nbits_max);
+
+ queue_entropy(r, q, nbits << ENTROPY_SHIFT);
+ return 0;
+}
+
/*
* Dequeue previously queued entropy and return the pool entropy
* watermark to be used in pool_entropy_delta().
@@ -950,6 +972,22 @@ static void dispatch_queued_entropy(struct entropy_store *r,
}
}

+/*
+ * Discard queued entropy. May be called when e.g. a write_pool()
+ * operation failed and the corresponding previously queued entropy
+ * should not get dispatched to the pool.
+ */
+static void discard_queued_entropy(struct entropy_store *r,
+ struct queued_entropy *q)
+{
+ unsigned long flags;
+ int pool_watermark;
+
+ spin_lock_irqsave(&r->lock, flags);
+ __dequeue_entropy(r, q, &pool_watermark);
+ spin_unlock_irqrestore(&r->lock, flags);
+}
+
/*
* Credit the entropy store with n bits of entropy.
* Use credit_entropy_bits_safe() if the value comes from userspace
@@ -2272,6 +2310,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
int size, ent_count;
int __user *p = (int __user *)arg;
int retval;
+ struct queued_entropy q = { 0 };

switch (cmd) {
case RNDGETENTCNT:
@@ -2285,7 +2324,11 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
return -EPERM;
if (get_user(ent_count, p))
return -EFAULT;
- return credit_entropy_bits_safe(&input_pool, ent_count);
+ retval = queue_entropy_bits_safe(&input_pool, &q, ent_count);
+ if (retval < 0)
+ return retval;
+ dispatch_queued_entropy(&input_pool, &q);
+ return 0;
case RNDADDENTROPY:
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
@@ -2295,11 +2338,17 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
return -EINVAL;
if (get_user(size, p++))
return -EFAULT;
+ retval = queue_entropy_bits_safe(&input_pool, &q, ent_count);
+ if (retval < 0)
+ return retval;
retval = write_pool(&input_pool, (const char __user *)p,
size);
- if (retval < 0)
+ if (retval < 0) {
+ discard_queued_entropy(&input_pool, &q);
return retval;
- return credit_entropy_bits_safe(&input_pool, ent_count);
+ }
+ discard_queued_entropy(&input_pool, &q);
+ return 0;
case RNDZAPENTCNT:
case RNDCLEARPOOL:
/*
--
2.26.2
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