[PATCH v5 REPOST 0/6] fix hw_random stuck

[PATCH v5 REPOST 0/6] fix hw_random stuck

[Amos Kong]

When I hotunplug a busy virtio-rng device or try to access
hwrng attributes in non-smp guest, it gets stuck.

My hotplug tests:

| test 0:
|   hotunplug rng device from qemu monitor
|
| test 1:
|   guest) # dd if=/dev/hwrng of=/dev/null &
|   hotunplug rng device from qemu monitor
|
| test 2:
|   guest) # dd if=/dev/random of=/dev/null &
|   hotunplug rng device from qemu monitor
|
| test 4:
|   guest) # dd if=/dev/hwrng of=/dev/null &
|   cat /sys/devices/virtual/misc/hw_random/rng_*
|
| test 5:
|   guest) # dd if=/dev/hwrng of=/dev/null
|   cancel dd process after 10 seconds
|   guest) # dd if=/dev/hwrng of=/dev/null &
|   hotunplug rng device from qemu monitor
|
| test 6:
|   use a fifo as rng backend, execute test 0 ~ 5 with no input of fifo

V5: reset cleanup_done flag, drop redundant init of reference count, use
    compiler barrier to prevent recording.
V4: update patch 4 to fix corrupt, decrease last reference for triggering
    the cleanup, fix unregister race pointed by Herbert
V3: initialize kref to 1
V2: added patch 2 to fix a deadlock, update current patch 3 to fix reference
    counting issue

Amos Kong (1):
  hw_random: move some code out mutex_lock for avoiding underlying
    deadlock

Rusty Russell (5):
  hw_random: place mutex around read functions and buffers.
  hw_random: use reference counts on each struct hwrng.
  hw_random: fix unregister race.
  hw_random: don't double-check old_rng.
  hw_random: don't init list element we're about to add to list.

 drivers/char/hw_random/core.c | 173 ++++++++++++++++++++++++++++++------------
 include/linux/hw_random.h     |   3 +
 2 files changed, 126 insertions(+), 50 deletions(-)

-- 
1.9.3

[PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.

[Amos Kong]

From: Rusty Russell <rusty@rustcorp.com.au>

There's currently a big lock around everything, and it means that we
can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current)
while the rng is reading.  This is a real problem when the rng is slow,
or blocked (eg. virtio_rng with qemu's default /dev/random backend)

This doesn't help (it leaves the current lock untouched), just adds a
lock to protect the read function and the static buffers, in preparation
for transition.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/char/hw_random/core.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..b1b6042 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -53,7 +53,10 @@
 static struct hwrng *current_rng;
 static struct task_struct *hwrng_fill;
 static LIST_HEAD(rng_list);
+/* Protects rng_list and current_rng */
 static DEFINE_MUTEX(rng_mutex);
+/* Protects rng read functions, data_avail, rng_buffer and rng_fillbuf */
+static DEFINE_MUTEX(reading_mutex);
 static int data_avail;
 static u8 *rng_buffer, *rng_fillbuf;
 static unsigned short current_quality;
@@ -81,7 +84,9 @@ static void add_early_randomness(struct hwrng *rng)
 	unsigned char bytes[16];
 	int bytes_read;
 
+	mutex_lock(&reading_mutex);
 	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
+	mutex_unlock(&reading_mutex);
 	if (bytes_read > 0)
 		add_device_randomness(bytes, bytes_read);
 }
@@ -128,6 +133,7 @@ static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
 			int wait) {
 	int present;
 
+	BUG_ON(!mutex_is_locked(&reading_mutex));
 	if (rng->read)
 		return rng->read(rng, (void *)buffer, size, wait);
 
@@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 			goto out_unlock;
 		}
 
+		mutex_lock(&reading_mutex);
 		if (!data_avail) {
 			bytes_read = rng_get_data(current_rng, rng_buffer,
 				rng_buffer_size(),
 				!(filp->f_flags & O_NONBLOCK));
 			if (bytes_read < 0) {
 				err = bytes_read;
-				goto out_unlock;
+				goto out_unlock_reading;
 			}
 			data_avail = bytes_read;
 		}
@@ -174,7 +181,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 		if (!data_avail) {
 			if (filp->f_flags & O_NONBLOCK) {
 				err = -EAGAIN;
-				goto out_unlock;
+				goto out_unlock_reading;
 			}
 		} else {
 			len = data_avail;
@@ -186,7 +193,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 			if (copy_to_user(buf + ret, rng_buffer + data_avail,
 								len)) {
 				err = -EFAULT;
-				goto out_unlock;
+				goto out_unlock_reading;
 			}
 
 			size -= len;
@@ -194,6 +201,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 		}
 
 		mutex_unlock(&rng_mutex);
+		mutex_unlock(&reading_mutex);
 
 		if (need_resched())
 			schedule_timeout_interruptible(1);
@@ -208,6 +216,9 @@ out:
 out_unlock:
 	mutex_unlock(&rng_mutex);
 	goto out;
+out_unlock_reading:
+	mutex_unlock(&reading_mutex);
+	goto out_unlock;
 }
 
 
@@ -348,13 +359,16 @@ static int hwrng_fillfn(void *unused)
 	while (!kthread_should_stop()) {
 		if (!current_rng)
 			break;
+		mutex_lock(&reading_mutex);
 		rc = rng_get_data(current_rng, rng_fillbuf,
 				  rng_buffer_size(), 1);
+		mutex_unlock(&reading_mutex);
 		if (rc <= 0) {
 			pr_warn("hwrng: no data available\n");
 			msleep_interruptible(10000);
 			continue;
 		}
+		/* Outside lock, sure, but y'know: randomness. */
 		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
 					   rc * current_quality * 8 >> 10);
 	}
-- 
1.9.3

Re: [PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.

[Dmitry Torokhov]

A bit late to a party, but:

On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@redhat.com> wrote:
> From: Rusty Russell <rusty@rustcorp.com.au>
>
> There's currently a big lock around everything, and it means that we
> can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current)
> while the rng is reading.  This is a real problem when the rng is slow,
> or blocked (eg. virtio_rng with qemu's default /dev/random backend)
>
> This doesn't help (it leaves the current lock untouched), just adds a
> lock to protect the read function and the static buffers, in preparation
> for transition.
>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> ---
...
>
> @@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
>                         goto out_unlock;
>                 }
>
> +               mutex_lock(&reading_mutex);

I think this breaks O_NONBLOCK: we have hwrng core thread that is
constantly pumps underlying rng for data; the thread takes the mutex
and calls rng_get_data() that blocks until RNG responds. This means
that even user specified O_NONBLOCK here we'll be waiting until
[hwrng] thread releases reading_mutex before we can continue.

>                 if (!data_avail) {
>                         bytes_read = rng_get_data(current_rng, rng_buffer,
>                                 rng_buffer_size(),
>                                 !(filp->f_flags & O_NONBLOCK));
>                         if (bytes_read < 0) {
>                                 err = bytes_read;
> -                               goto out_unlock;
> +                               goto out_unlock_reading;
>                         }
>                         data_avail = bytes_read;
>                 }

Thanks.

-- 
Dmitry

Re: [PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.

[Pankaj Gupta]

> 
> A bit late to a party, but:
> 
> On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@redhat.com> wrote:
> > From: Rusty Russell <rusty@rustcorp.com.au>
> >
> > There's currently a big lock around everything, and it means that we
> > can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current)
> > while the rng is reading.  This is a real problem when the rng is slow,
> > or blocked (eg. virtio_rng with qemu's default /dev/random backend)
> >
> > This doesn't help (it leaves the current lock untouched), just adds a
> > lock to protect the read function and the static buffers, in preparation
> > for transition.
> >
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > ---
> ...
> >
> > @@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp, char
> > __user *buf,
> >                         goto out_unlock;
> >                 }
> >
> > +               mutex_lock(&reading_mutex);
> 
> I think this breaks O_NONBLOCK: we have hwrng core thread that is
> constantly pumps underlying rng for data; the thread takes the mutex
> and calls rng_get_data() that blocks until RNG responds. This means
> that even user specified O_NONBLOCK here we'll be waiting until
> [hwrng] thread releases reading_mutex before we can continue.

I think for 'virtio_rng' for 'O_NON_BLOCK' 'rng_get_data' returns
without waiting for data which can let mutex to be  used by other 
threads waiting if any?

rng_dev_read
  rng_get_data
    virtio_read
  
static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
{
        int ret;
        struct virtrng_info *vi = (struct virtrng_info *)rng->priv;

        if (vi->hwrng_removed)
                return -ENODEV;

        if (!vi->busy) {
                vi->busy = true;
                init_completion(&vi->have_data);
                register_buffer(vi, buf, size);
        }

        if (!wait)
                return 0;

        ret = wait_for_completion_killable(&vi->have_data);
        if (ret < 0)
                return ret;

        vi->busy = false;

        return vi->data_avail;
}

> 
> >                 if (!data_avail) {
> >                         bytes_read = rng_get_data(current_rng, rng_buffer,
> >                                 rng_buffer_size(),
> >                                 !(filp->f_flags & O_NONBLOCK));
> >                         if (bytes_read < 0) {
> >                                 err = bytes_read;
> > -                               goto out_unlock;
> > +                               goto out_unlock_reading;
> >                         }
> >                         data_avail = bytes_read;
> >                 }
> 
> Thanks.
> 
> --
> Dmitry
> 

Re: [PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.

[Dmitry Torokhov]

On Tue, Sep 26, 2017 at 02:36:57AM -0400, Pankaj Gupta wrote:
> 
> > 
> > A bit late to a party, but:
> > 
> > On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@redhat.com> wrote:
> > > From: Rusty Russell <rusty@rustcorp.com.au>
> > >
> > > There's currently a big lock around everything, and it means that we
> > > can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current)
> > > while the rng is reading.  This is a real problem when the rng is slow,
> > > or blocked (eg. virtio_rng with qemu's default /dev/random backend)
> > >
> > > This doesn't help (it leaves the current lock untouched), just adds a
> > > lock to protect the read function and the static buffers, in preparation
> > > for transition.
> > >
> > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > > ---
> > ...
> > >
> > > @@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp, char
> > > __user *buf,
> > >                         goto out_unlock;
> > >                 }
> > >
> > > +               mutex_lock(&reading_mutex);
> > 
> > I think this breaks O_NONBLOCK: we have hwrng core thread that is
> > constantly pumps underlying rng for data; the thread takes the mutex
> > and calls rng_get_data() that blocks until RNG responds. This means
> > that even user specified O_NONBLOCK here we'll be waiting until
> > [hwrng] thread releases reading_mutex before we can continue.
> 
> I think for 'virtio_rng' for 'O_NON_BLOCK' 'rng_get_data' returns
> without waiting for data which can let mutex to be  used by other 
> threads waiting if any?
> 
> rng_dev_read
>   rng_get_data
>     virtio_read

As I said in the paragraph above the code that potentially holds the
mutex for long time is the thread in hwrng core: hwrng_fillfn(). As it
calls rng_get_data() with "wait" argument == 1 it may block while
holding reading_mutex, which, in turn, will block rng_dev_read(), even
if it was called with O_NONBLOCK.

Thanks.

-- 
Dmitry

Re: [PATCH v5 REPOST 1/6] hw_random: place mutex around read functions and buffers.

[Pankaj Gupta]

> 
> On Tue, Sep 26, 2017 at 02:36:57AM -0400, Pankaj Gupta wrote:
> > 
> > > 
> > > A bit late to a party, but:
> > > 
> > > On Mon, Dec 8, 2014 at 12:50 AM, Amos Kong <akong@redhat.com> wrote:
> > > > From: Rusty Russell <rusty@rustcorp.com.au>
> > > >
> > > > There's currently a big lock around everything, and it means that we
> > > > can't query sysfs (eg /sys/devices/virtual/misc/hw_random/rng_current)
> > > > while the rng is reading.  This is a real problem when the rng is slow,
> > > > or blocked (eg. virtio_rng with qemu's default /dev/random backend)
> > > >
> > > > This doesn't help (it leaves the current lock untouched), just adds a
> > > > lock to protect the read function and the static buffers, in
> > > > preparation
> > > > for transition.
> > > >
> > > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > > > ---
> > > ...
> > > >
> > > > @@ -160,13 +166,14 @@ static ssize_t rng_dev_read(struct file *filp,
> > > > char
> > > > __user *buf,
> > > >                         goto out_unlock;
> > > >                 }
> > > >
> > > > +               mutex_lock(&reading_mutex);
> > > 
> > > I think this breaks O_NONBLOCK: we have hwrng core thread that is
> > > constantly pumps underlying rng for data; the thread takes the mutex
> > > and calls rng_get_data() that blocks until RNG responds. This means
> > > that even user specified O_NONBLOCK here we'll be waiting until
> > > [hwrng] thread releases reading_mutex before we can continue.
> > 
> > I think for 'virtio_rng' for 'O_NON_BLOCK' 'rng_get_data' returns
> > without waiting for data which can let mutex to be  used by other
> > threads waiting if any?
> > 
> > rng_dev_read
> >   rng_get_data
> >     virtio_read
> 
> As I said in the paragraph above the code that potentially holds the
> mutex for long time is the thread in hwrng core: hwrng_fillfn(). As it
> calls rng_get_data() with "wait" argument == 1 it may block while
> holding reading_mutex, which, in turn, will block rng_dev_read(), even
> if it was called with O_NONBLOCK.

yes, 'hwrng_fillfn' does not consider O_NONBLOCK and can result in mutex wait
for other tasks. What if we pass zero for wait to 'hwrng_fill' to return early 
if there is no data?

--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -403,7 +403,7 @@ static int hwrng_fillfn(void *unused)
                        break;
                mutex_lock(&reading_mutex);
                rc = rng_get_data(rng, rng_fillbuf,
-                                 rng_buffer_size(), 1);
+                                 rng_buffer_size(), 0);
                mutex_unlock(&reading_mutex);
                put_rng(rng);
                if (rc <= 0) {

Thanks,
Pankaj

> 
> Thanks.
> 
> --
> Dmitry
> 

[PATCH v5 REPOST 2/6] hw_random: move some code out mutex_lock for avoiding underlying deadlock

[Amos Kong]

In next patch, we use reference counting for each struct hwrng,
changing reference count also needs to take mutex_lock. Before
releasing the lock, if we try to stop a kthread that waits to
take the lock to reduce the referencing count, deadlock will
occur.

Signed-off-by: Amos Kong <akong@redhat.com>
---
 drivers/char/hw_random/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index b1b6042..a0905c8 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -474,12 +474,12 @@ void hwrng_unregister(struct hwrng *rng)
 		}
 	}
 	if (list_empty(&rng_list)) {
+		mutex_unlock(&rng_mutex);
 		unregister_miscdev();
 		if (hwrng_fill)
 			kthread_stop(hwrng_fill);
-	}
-
-	mutex_unlock(&rng_mutex);
+	} else
+		mutex_unlock(&rng_mutex);
 }
 EXPORT_SYMBOL_GPL(hwrng_unregister);
 
-- 
1.9.3

[PATCH v5 REPOST 3/6] hw_random: use reference counts on each struct hwrng.

[Amos Kong]

From: Rusty Russell <rusty@rustcorp.com.au>

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.

v5: drop redundant kref_init()
v4: decrease last reference for triggering the cleanup
v3: initialize kref (thanks Amos Kong)
v2: fix missing put_rng() on exit path (thanks Amos Kong)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Amos Kong <akong@redhat.com>
---
 drivers/char/hw_random/core.c | 135 ++++++++++++++++++++++++++++--------------
 include/linux/hw_random.h     |   2 +
 2 files changed, 94 insertions(+), 43 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index a0905c8..83516cb 100644
--- 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;
 }
@@ -357,12 +404,16 @@ 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);
@@ -423,14 +474,13 @@ 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;
 		}
 	}
@@ -457,22 +507,21 @@ 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
--- 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. */
-- 
1.9.3

[PATCH v5 REPOST 4/6] hw_random: fix unregister race.

[Amos Kong]

From: Rusty Russell <rusty@rustcorp.com.au>

The previous patch added one potential problem: we can still be
reading from a hwrng when it's unregistered.  Add a wait for zero
in the hwrng_unregister path.

v5: reset cleanup_done flag, use compiler barrier to prevent recording.
v4: add cleanup_done flag to insure that cleanup is done

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Amos Kong <akong@redhat.com>
---
 drivers/char/hw_random/core.c | 12 ++++++++++++
 include/linux/hw_random.h     |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 83516cb..067270b 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -60,6 +60,7 @@ static DEFINE_MUTEX(rng_mutex);
 static DEFINE_MUTEX(reading_mutex);
 static int data_avail;
 static u8 *rng_buffer, *rng_fillbuf;
+static DECLARE_WAIT_QUEUE_HEAD(rng_done);
 static unsigned short current_quality;
 static unsigned short default_quality; /* = 0; default to "off" */
 
@@ -98,6 +99,11 @@ static inline void cleanup_rng(struct kref *kref)
 
 	if (rng->cleanup)
 		rng->cleanup(rng);
+
+	/* cleanup_done should be updated after cleanup finishes */
+	smp_wmb();
+	rng->cleanup_done = true;
+	wake_up_all(&rng_done);
 }
 
 static void set_current_rng(struct hwrng *rng)
@@ -498,6 +504,8 @@ int hwrng_register(struct hwrng *rng)
 		add_early_randomness(rng);
 	}
 
+	rng->cleanup_done = false;
+
 out_unlock:
 	mutex_unlock(&rng_mutex);
 out:
@@ -529,6 +537,10 @@ void hwrng_unregister(struct hwrng *rng)
 			kthread_stop(hwrng_fill);
 	} else
 		mutex_unlock(&rng_mutex);
+
+	/* Just in case rng is reading right now, wait. */
+	wait_event(rng_done, rng->cleanup_done &&
+		   atomic_read(&rng->ref.refcount) == 0);
 }
 EXPORT_SYMBOL_GPL(hwrng_unregister);
 
diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index c212e71..7832e50 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -46,6 +46,7 @@ struct hwrng {
 	/* internal. */
 	struct list_head list;
 	struct kref ref;
+	bool cleanup_done;
 };
 
 /** Register a new Hardware Random Number Generator driver. */
-- 
1.9.3

[PATCH v5 REPOST 5/6] hw_random: don't double-check old_rng.

[Amos Kong]

From: Rusty Russell <rusty@rustcorp.com.au>

Interesting anti-pattern.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/char/hw_random/core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 067270b..a9286bf 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -476,14 +476,13 @@ int hwrng_register(struct hwrng *rng)
 	}
 
 	old_rng = current_rng;
+	err = 0;
 	if (!old_rng) {
 		err = hwrng_init(rng);
 		if (err)
 			goto out_unlock;
 		set_current_rng(rng);
-	}
-	err = 0;
-	if (!old_rng) {
+
 		err = register_miscdev();
 		if (err) {
 			drop_current_rng();
-- 
1.9.3

[PATCH v5 REPOST 6/6] hw_random: don't init list element we're about to add to list.

[Amos Kong]

From: Rusty Russell <rusty@rustcorp.com.au>

Another interesting anti-pattern.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/char/hw_random/core.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index a9286bf..4d13ac5 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -489,7 +489,6 @@ int hwrng_register(struct hwrng *rng)
 			goto out_unlock;
 		}
 	}
-	INIT_LIST_HEAD(&rng->list);
 	list_add_tail(&rng->list, &rng_list);
 
 	if (old_rng && !rng->init) {
-- 
1.9.3

Re: [PATCH v5 REPOST 0/6] fix hw_random stuck

[Herbert Xu]

On Mon, Dec 08, 2014 at 04:50:34PM +0800, Amos Kong wrote:
> When I hotunplug a busy virtio-rng device or try to access
> hwrng attributes in non-smp guest, it gets stuck.
> 
> My hotplug tests:
> 
> | test 0:
> |   hotunplug rng device from qemu monitor
> |
> | test 1:
> |   guest) # dd if=/dev/hwrng of=/dev/null &
> |   hotunplug rng device from qemu monitor
> |
> | test 2:
> |   guest) # dd if=/dev/random of=/dev/null &
> |   hotunplug rng device from qemu monitor
> |
> | test 4:
> |   guest) # dd if=/dev/hwrng of=/dev/null &
> |   cat /sys/devices/virtual/misc/hw_random/rng_*
> |
> | test 5:
> |   guest) # dd if=/dev/hwrng of=/dev/null
> |   cancel dd process after 10 seconds
> |   guest) # dd if=/dev/hwrng of=/dev/null &
> |   hotunplug rng device from qemu monitor
> |
> | test 6:
> |   use a fifo as rng backend, execute test 0 ~ 5 with no input of fifo

All applied.  Thanks a lot!
-- 
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