Re: [PATCH] Input: uinput: Fix race condition on 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 never calling wait_event_*() for non-blocking FDs hence we
will never sleep in the waitq here.

Signed-off-by: David Herrmann <redacted>
---
 drivers/input/misc/uinput.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index 7360568..1526814 100644
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c

@@ -460,13 +460,15 @@ static ssize_t uinput_read(struct file *file, char __user *buffer, size_t count,
 	if (udev->state != UIST_CREATED)
 		return -ENODEV;
 
-	if (udev->head == udev->tail && (file->f_flags & O_NONBLOCK))
-		return -EAGAIN;
-
-	retval = wait_event_interruptible(udev->waitq,
+	if (file->f_flags & O_NONBLOCK) {
+		if (udev->head == udev->tail)
+			return -EAGAIN;
+	} else {
+		retval = wait_event_interruptible(udev->waitq,
 			udev->head != udev->tail || udev->state != UIST_CREATED);
-	if (retval)
-		return retval;
+		if (retval)
+			return retval;
+	}
 
 	retval = mutex_lock_interruptible(&udev->mutex);
 	if (retval)