misc: mic: fix possible signed underflow (undefined behavior) in userspace API

iovcnt is declared as a signed integer in both the userspace API and
as a local variable in mic_virtio.c. The while() loop in mic_virtio.c
iterates until the local variable iovcnt reaches the value 0. If
userspace passes e.g. INT_MIN as iovcnt field, this loop then appears
to depend on an undefined behavior (signed underflow) to complete.
The fix is to use unsigned integers in both the userspace API and
the local variable.

This issue was reported @ https://lkml.org/lkml/2014/1/10/10

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Sudeep Dutt <sudeep.dutt@intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mic/host/mic_virtio.c

@@ -156,7 +156,8 @@ static int mic_vringh_copy(struct mic_vdev *mvdev, struct vringh_kiov *iov,
 static int _mic_virtio_copy(struct mic_vdev *mvdev,
 	struct mic_copy_desc *copy)
 {
-	int ret = 0, iovcnt = copy->iovcnt;
+	int ret = 0;
+	u32 iovcnt = copy->iovcnt;
 	struct iovec iov;
 	struct iovec __user *u_iov = copy->iov;
 	void __user *ubuf = NULL;

include/uapi/linux/mic_ioctl.h

@@ -39,7 +39,7 @@ struct mic_copy_desc {
 #else
 	struct iovec *iov;
 #endif
-	int iovcnt;
+	__u32 iovcnt;
 	__u8 vr_idx;
 	__u8 update_used;
 	__u32 out_len;