[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)

-- 
1.7.9.4

On Fri, Mar 16, 2012 at 09:04:29PM +0100, David Herrmann wrote:

makes sense, patch looks good
Acked-by: Aristeu Rozanski <redacted>

-- 
Aristeu

Hi Dmitry

On Mon, Mar 19, 2012 at 6:25 PM, Aristeu Rozanski
[off-list ref] wrote:

On Fri, Mar 16, 2012 at 09:04:29PM +0100, David Herrmann wrote:

quoted

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)

makes sense, patch looks good
Acked-by: Aristeu Rozanski <redacted>

Ping (and the other uinput fix ;)).

Regards
David
--
To unsubscribe from this list: send the line "unsubscribe linux-input" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html