Input: uinput - fix race that can block nonblocking read

Consider two threads calling read() on the same uinput-fd, both
non-blocking. Assume there is data-available so both will simultaneously
pass:
	udev->head == udev->tail

Then the first thread goes to sleep and the second one pops the message
from the queue. Now assume udev->head == udev->tail. If the first thread
wakes up it will call wait_event_*() and sleep in the waitq. This
effectively turns the non-blocking FD into a blocking one.

We fix this by attempting to fetch events from the queue first and only
if we fail to retrieve any events we either return -EAGAIN (in case of
non-blocing read) or wait until there are more events.

This also fixes incorrect return code (we were returning 0 instead of
 -EAGAIN for non-blocking reads) when an event is "stolen" by another
thread. Blocking reads will now continue to wait instead of returning 0
in this scenario.

Count of 0 continues to be a special case, as per spec: we will check for
device existence and whether there are events in the queue, but no events
will be actually retrieved.

Reported-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

1 files changed, 44 insertions, 30 deletions

diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index e74ed9cc6371..1719554fe194 100644
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -439,6 +439,9 @@ static ssize_t uinput_write(struct file *file, const char __user *buffer, size_t
         struct uinput_device *udev = file->private_data;
         int retval;
 
+        if (count == 0)
+                return 0;
+
         retval = mutex_lock_interruptible(&udev->mutex);
         if (retval)
                 return retval;
@@ -470,48 +473,59 @@ static bool uinput_fetch_next_event(struct uinput_device *udev,
         return have_event;
 }
 
-static ssize_t uinput_read(struct file *file, char __user *buffer, size_t count, loff_t *ppos)
+static ssize_t uinput_events_to_user(struct uinput_device *udev,
+                                     char __user *buffer, size_t count)
 {
-        struct uinput_device *udev = file->private_data;
         struct input_event event;
-        int retval = 0;
+        size_t read = 0;
+        int error = 0;
 
-        if (count != 0 && count < input_event_size())
-                return -EINVAL;
+        while (read + input_event_size() <= count &&
+               uinput_fetch_next_event(udev, &event)) {
 
-        if (udev->state != UIST_CREATED)
-                return -ENODEV;
+                if (input_event_to_user(buffer + read, &event)) {
+                        error = -EFAULT;
+                        break;
+                }
 
-        if (udev->head == udev->tail && (file->f_flags & O_NONBLOCK))
-                return -EAGAIN;
+                read += input_event_size();
+        }
 
-        retval = wait_event_interruptible(udev->waitq,
-                        udev->head != udev->tail || udev->state != UIST_CREATED);
-        if (retval)
-                return retval;
+        return read ?: error;
+}
 
-        retval = mutex_lock_interruptible(&udev->mutex);
-        if (retval)
-                return retval;
+static ssize_t uinput_read(struct file *file, char __user *buffer,
+                           size_t count, loff_t *ppos)
+{
+        struct uinput_device *udev = file->private_data;
+        ssize_t retval;
 
-        if (udev->state != UIST_CREATED) {
-                retval = -ENODEV;
-                goto out;
-        }
+        if (count != 0 && count < input_event_size())
+                return -EINVAL;
 
-        while (retval + input_event_size() <= count &&
-               uinput_fetch_next_event(udev, &event)) {
+        do {
+                retval = mutex_lock_interruptible(&udev->mutex);
+                if (retval)
+                        return retval;
 
-                if (input_event_to_user(buffer + retval, &event)) {
-                        retval = -EFAULT;
-                        goto out;
-                }
+                if (udev->state != UIST_CREATED)
+                        retval = -ENODEV;
+                else if (udev->head == udev->tail &&
+                         (file->f_flags & O_NONBLOCK))
+                        retval = -EAGAIN;
+                else
+                        retval = uinput_events_to_user(udev, buffer, count);
 
-                retval += input_event_size();
-        }
+                mutex_unlock(&udev->mutex);
 
- out:
-        mutex_unlock(&udev->mutex);
+                if (retval || count == 0)
+                        break;
+
+                if (!(file->f_flags & O_NONBLOCK))
+                        retval = wait_event_interruptible(udev->waitq,
+                                                  udev->head != udev->tail ||
+                                                  udev->state != UIST_CREATED);
+        } while (retval == 0);
 
         return retval;
 }