hwrng: use reference counts on each struct hwrng.

current_rng holds one reference, and we bump it every time we want
to do a read from it.

This means we only hold the rng_mutex to grab or drop a reference,
so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
block on read of /dev/hwrng.

Using a kref is overkill (we're always under the rng_mutex), but
a standard pattern.

This also solves the problem that the hwrng_fillfn thread was
accessing current_rng without a lock, which could change (eg. to NULL)
underneath it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Amos Kong <akong@redhat.com>
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 b4c0e87..089c18d 100644 (file)
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -42,6 +42,7 @@
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/random.h>
+#include <linux/err.h>
 #include <asm/uaccess.h>
 
 
@@ -91,6 +92,60 @@ static void add_early_randomness(struct hwrng *rng)
                add_device_randomness(bytes, bytes_read);
 }
 
+static inline void cleanup_rng(struct kref *kref)
+{
+       struct hwrng *rng = container_of(kref, struct hwrng, ref);
+
+       if (rng->cleanup)
+               rng->cleanup(rng);
+}
+
+static void set_current_rng(struct hwrng *rng)
+{
+       BUG_ON(!mutex_is_locked(&rng_mutex));
+       kref_get(&rng->ref);
+       current_rng = rng;
+}
+
+static void drop_current_rng(void)
+{
+       BUG_ON(!mutex_is_locked(&rng_mutex));
+       if (!current_rng)
+               return;
+
+       /* decrease last reference for triggering the cleanup */
+       kref_put(&current_rng->ref, cleanup_rng);
+       current_rng = NULL;
+}
+
+/* Returns ERR_PTR(), NULL or refcounted hwrng */
+static struct hwrng *get_current_rng(void)
+{
+       struct hwrng *rng;
+
+       if (mutex_lock_interruptible(&rng_mutex))
+               return ERR_PTR(-ERESTARTSYS);
+
+       rng = current_rng;
+       if (rng)
+               kref_get(&rng->ref);
+
+       mutex_unlock(&rng_mutex);
+       return rng;
+}
+
+static void put_rng(struct hwrng *rng)
+{
+       /*
+        * Hold rng_mutex here so we serialize in case they set_current_rng
+        * on rng again immediately.
+        */
+       mutex_lock(&rng_mutex);
+       if (rng)
+               kref_put(&rng->ref, cleanup_rng);
+       mutex_unlock(&rng_mutex);
+}
+
 static inline int hwrng_init(struct hwrng *rng)
 {
        if (rng->init) {
@@ -113,12 +168,6 @@ static inline int hwrng_init(struct hwrng *rng)
        return 0;
 }
 
-static inline void hwrng_cleanup(struct hwrng *rng)
-{
-       if (rng && rng->cleanup)
-               rng->cleanup(rng);
-}
-
 static int rng_dev_open(struct inode *inode, struct file *filp)
 {
        /* enforce read-only access to this chrdev */
@@ -154,21 +203,22 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
        ssize_t ret = 0;
        int err = 0;
        int bytes_read, len;
+       struct hwrng *rng;
 
        while (size) {
-               if (mutex_lock_interruptible(&rng_mutex)) {
-                       err = -ERESTARTSYS;
+               rng = get_current_rng();
+               if (IS_ERR(rng)) {
+                       err = PTR_ERR(rng);
                        goto out;
                }
-
-               if (!current_rng) {
+               if (!rng) {
                        err = -ENODEV;
-                       goto out_unlock;
+                       goto out;
                }
 
                mutex_lock(&reading_mutex);
                if (!data_avail) {
-                       bytes_read = rng_get_data(current_rng, rng_buffer,
+                       bytes_read = rng_get_data(rng, rng_buffer,
                                rng_buffer_size(),
                                !(filp->f_flags & O_NONBLOCK));
                        if (bytes_read < 0) {
@@ -200,8 +250,8 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
                        ret += len;
                }
 
-               mutex_unlock(&rng_mutex);
                mutex_unlock(&reading_mutex);
+               put_rng(rng);
 
                if (need_resched())
                        schedule_timeout_interruptible(1);
@@ -213,12 +263,11 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
        }
 out:
        return ret ? : err;
-out_unlock:
-       mutex_unlock(&rng_mutex);
-       goto out;
+
 out_unlock_reading:
        mutex_unlock(&reading_mutex);
-       goto out_unlock;
+       put_rng(rng);
+       goto out;
 }
 
 
@@ -257,8 +306,8 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
                        err = hwrng_init(rng);
                        if (err)
                                break;
-                       hwrng_cleanup(current_rng);
-                       current_rng = rng;
+                       drop_current_rng();
+                       set_current_rng(rng);
                        err = 0;
                        break;
                }
@@ -272,17 +321,15 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
                                       struct device_attribute *attr,
                                       char *buf)
 {
-       int err;
        ssize_t ret;
-       const char *name = "none";
+       struct hwrng *rng;
 
-       err = mutex_lock_interruptible(&rng_mutex);
-       if (err)
-               return -ERESTARTSYS;
-       if (current_rng)
-               name = current_rng->name;
-       ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
-       mutex_unlock(&rng_mutex);
+       rng = get_current_rng();
+       if (IS_ERR(rng))
+               return PTR_ERR(rng);
+
+       ret = snprintf(buf, PAGE_SIZE, "%s\n", rng ? rng->name : "none");
+       put_rng(rng);
 
        return ret;
 }
@@ -353,12 +400,16 @@ static int hwrng_fillfn(void *unused)
        long rc;
 
        while (!kthread_should_stop()) {
-               if (!current_rng)
+               struct hwrng *rng;
+
+               rng = get_current_rng();
+               if (IS_ERR(rng) || !rng)
                        break;
                mutex_lock(&reading_mutex);
-               rc = rng_get_data(current_rng, rng_fillbuf,
+               rc = rng_get_data(rng, rng_fillbuf,
                                  rng_buffer_size(), 1);
                mutex_unlock(&reading_mutex);
+               put_rng(rng);
                if (rc <= 0) {
                        pr_warn("hwrng: no data available\n");
                        msleep_interruptible(10000);
@@ -419,14 +470,13 @@ int hwrng_register(struct hwrng *rng)
                err = hwrng_init(rng);
                if (err)
                        goto out_unlock;
-               current_rng = rng;
+               set_current_rng(rng);
        }
        err = 0;
        if (!old_rng) {
                err = register_miscdev();
                if (err) {
-                       hwrng_cleanup(rng);
-                       current_rng = NULL;
+                       drop_current_rng();
                        goto out_unlock;
                }
        }
@@ -453,22 +503,21 @@ EXPORT_SYMBOL_GPL(hwrng_register);
 
 void hwrng_unregister(struct hwrng *rng)
 {
-       int err;
-
        mutex_lock(&rng_mutex);
 
        list_del(&rng->list);
        if (current_rng == rng) {
-               hwrng_cleanup(rng);
-               if (list_empty(&rng_list)) {
-                       current_rng = NULL;
-               } else {
-                       current_rng = list_entry(rng_list.prev, struct hwrng, list);
-                       err = hwrng_init(current_rng);
-                       if (err)
-                               current_rng = NULL;
+               drop_current_rng();
+               if (!list_empty(&rng_list)) {
+                       struct hwrng *tail;
+
+                       tail = list_entry(rng_list.prev, struct hwrng, list);
+
+                       if (hwrng_init(tail) == 0)
+                               set_current_rng(tail);
                }
        }
+
        if (list_empty(&rng_list)) {
                mutex_unlock(&rng_mutex);
                unregister_miscdev();

diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index 914bb08..c212e71 100644 (file)
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -14,6 +14,7 @@
 
 #include <linux/types.h>
 #include <linux/list.h>
+#include <linux/kref.h>
 
 /**
  * struct hwrng - Hardware Random Number Generator driver
@@ -44,6 +45,7 @@ struct hwrng {
 
        /* internal. */
        struct list_head list;
+       struct kref ref;
 };
 
 /** Register a new Hardware Random Number Generator driver. */