patch "staging: vchiq_arm: fix compat VCHIQ_IOC_AWAIT_COMPLETION" added to staging-linus

This is a note to let you know that I've just added the patch titled

staging: vchiq_arm: fix compat VCHIQ_IOC_AWAIT_COMPLETION

to my staging git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
in the staging-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From: Ben Wolsieffer <[email protected]>
Date: Sat, 3 Nov 2018 19:32:20 -0400
Subject: staging: vchiq_arm: fix compat VCHIQ_IOC_AWAIT_COMPLETION

The compatibility ioctl wrapper for VCHIQ_IOC_AWAIT_COMPLETION assumes that the native ioctl always uses a message buffer and decrements msgbufcount. Certain message types do not use a message buffer and in this case msgbufcount is not decremented, and completion->header for the message is NULL. Because the wrapper unconditionally decrements msgbufcount, the calling process may assume that a message buffer has been used even when it has not.

This results in a memory leak in the userspace code that interfaces with this driver. When msgbufcount is decremented, the userspace code assumes that the buffer can be freed though the reference in completion->header, which cannot happen when the reference is NULL.

This patch causes the wrapper to only decrement msgbufcount when the native ioctl decrements it. Note that we cannot simply copy the native ioctl's value of msgbufcount, because the wrapper only retrieves messages from the native ioctl one at a time, while userspace may request multiple messages.

See https://github.com/raspberrypi/linux/pull/2703 for more discussion of this patch.

---
 .../staging/vc04_services/interface/vchiq_arm/vchiq_arm.c  | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index ea789376de0f..45de21c210c1 100644
@@ -1795,6 +1795,7 @@ vchiq_compat_ioctl_await_completion(struct file *file,
 	struct vchiq_await_completion32 args32;
 	struct vchiq_completion_data32 completion32;
 	unsigned int *msgbufcount32;
+	unsigned int msgbufcount_native;
 	compat_uptr_t msgbuf32;
 	void *msgbuf;
 	void **msgbufptr;

@@ -1906,7 +1907,11 @@ vchiq_compat_ioctl_await_completion(struct file *file,
 			 sizeof(completion32)))
 		return -EFAULT;
 
-	args32.msgbufcount--;
+	if (get_user(msgbufcount_native, &args->msgbufcount))
+		return -EFAULT;
+
+	if (!msgbufcount_native)
+		args32.msgbufcount--;
 
 	msgbufcount32 =
 		&((struct vchiq_await_completion32 __user *)arg)->msgbufcount;

2.19.1