usb: usbmon: Read text within supplied buffer size

This change fixes buffer overflows and silent data corruption with the
usbmon device driver text file read operations.

Signed-off-by: Fredrik Noring <noring@nocrew.org>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/mon/mon_text.c

@@ -85,6 +85,8 @@ struct mon_reader_text {
 
 	wait_queue_head_t wait;
 	int printf_size;
+	size_t printf_offset;
+	size_t printf_togo;
 	char *printf_buf;
 	struct mutex printf_lock;
 
@@ -376,75 +378,103 @@ err_alloc:
 	return rc;
 }
 
-/*
- * For simplicity, we read one record in one system call and throw out
- * what does not fit. This means that the following does not work:
- *   dd if=/dbg/usbmon/0t bs=10
- * Also, we do not allow seeks and do not bother advancing the offset.
- */
+static ssize_t mon_text_copy_to_user(struct mon_reader_text *rp,
+    char __user * const buf, const size_t nbytes)
+{
+	const size_t togo = min(nbytes, rp->printf_togo);
+
+	if (copy_to_user(buf, &rp->printf_buf[rp->printf_offset], togo))
+		return -EFAULT;
+	rp->printf_togo -= togo;
+	rp->printf_offset += togo;
+	return togo;
+}
+
+/* ppos is not advanced since the llseek operation is not permitted. */
 static ssize_t mon_text_read_t(struct file *file, char __user *buf,
-				size_t nbytes, loff_t *ppos)
+    size_t nbytes, loff_t *ppos)
 {
 	struct mon_reader_text *rp = file->private_data;
 	struct mon_event_text *ep;
 	struct mon_text_ptr ptr;
+	ssize_t ret;
 
-	ep = mon_text_read_wait(rp, file);
-	if (IS_ERR(ep))
-		return PTR_ERR(ep);
 	mutex_lock(&rp->printf_lock);
-	ptr.cnt = 0;
-	ptr.pbuf = rp->printf_buf;
-	ptr.limit = rp->printf_size;
-
-	mon_text_read_head_t(rp, &ptr, ep);
-	mon_text_read_statset(rp, &ptr, ep);
-	ptr.cnt += snprintf(ptr.pbuf + ptr.cnt, ptr.limit - ptr.cnt,
-	    " %d", ep->length);
-	mon_text_read_data(rp, &ptr, ep);
-
-	if (copy_to_user(buf, rp->printf_buf, ptr.cnt))
-		ptr.cnt = -EFAULT;
+
+	if (rp->printf_togo == 0) {
+
+		ep = mon_text_read_wait(rp, file);
+		if (IS_ERR(ep)) {
+			mutex_unlock(&rp->printf_lock);
+			return PTR_ERR(ep);
+		}
+		ptr.cnt = 0;
+		ptr.pbuf = rp->printf_buf;
+		ptr.limit = rp->printf_size;
+
+		mon_text_read_head_t(rp, &ptr, ep);
+		mon_text_read_statset(rp, &ptr, ep);
+		ptr.cnt += snprintf(ptr.pbuf + ptr.cnt, ptr.limit - ptr.cnt,
+		    " %d", ep->length);
+		mon_text_read_data(rp, &ptr, ep);
+
+		rp->printf_togo = ptr.cnt;
+		rp->printf_offset = 0;
+
+		kmem_cache_free(rp->e_slab, ep);
+	}
+
+	ret = mon_text_copy_to_user(rp, buf, nbytes);
 	mutex_unlock(&rp->printf_lock);
-	kmem_cache_free(rp->e_slab, ep);
-	return ptr.cnt;
+	return ret;
 }
 
+/* ppos is not advanced since the llseek operation is not permitted. */
 static ssize_t mon_text_read_u(struct file *file, char __user *buf,
-				size_t nbytes, loff_t *ppos)
+    size_t nbytes, loff_t *ppos)
 {
 	struct mon_reader_text *rp = file->private_data;
 	struct mon_event_text *ep;
 	struct mon_text_ptr ptr;
+	ssize_t ret;
 
-	ep = mon_text_read_wait(rp, file);
-	if (IS_ERR(ep))
-		return PTR_ERR(ep);
 	mutex_lock(&rp->printf_lock);
-	ptr.cnt = 0;
-	ptr.pbuf = rp->printf_buf;
-	ptr.limit = rp->printf_size;
 
-	mon_text_read_head_u(rp, &ptr, ep);
-	if (ep->type == 'E') {
-		mon_text_read_statset(rp, &ptr, ep);
-	} else if (ep->xfertype == USB_ENDPOINT_XFER_ISOC) {
-		mon_text_read_isostat(rp, &ptr, ep);
-		mon_text_read_isodesc(rp, &ptr, ep);
-	} else if (ep->xfertype == USB_ENDPOINT_XFER_INT) {
-		mon_text_read_intstat(rp, &ptr, ep);
-	} else {
-		mon_text_read_statset(rp, &ptr, ep);
+	if (rp->printf_togo == 0) {
+
+		ep = mon_text_read_wait(rp, file);
+		if (IS_ERR(ep)) {
+			mutex_unlock(&rp->printf_lock);
+			return PTR_ERR(ep);
+		}
+		ptr.cnt = 0;
+		ptr.pbuf = rp->printf_buf;
+		ptr.limit = rp->printf_size;
+
+		mon_text_read_head_u(rp, &ptr, ep);
+		if (ep->type == 'E') {
+			mon_text_read_statset(rp, &ptr, ep);
+		} else if (ep->xfertype == USB_ENDPOINT_XFER_ISOC) {
+			mon_text_read_isostat(rp, &ptr, ep);
+			mon_text_read_isodesc(rp, &ptr, ep);
+		} else if (ep->xfertype == USB_ENDPOINT_XFER_INT) {
+			mon_text_read_intstat(rp, &ptr, ep);
+		} else {
+			mon_text_read_statset(rp, &ptr, ep);
+		}
+		ptr.cnt += snprintf(ptr.pbuf + ptr.cnt, ptr.limit - ptr.cnt,
+		    " %d", ep->length);
+		mon_text_read_data(rp, &ptr, ep);
+
+		rp->printf_togo = ptr.cnt;
+		rp->printf_offset = 0;
+
+		kmem_cache_free(rp->e_slab, ep);
 	}
-	ptr.cnt += snprintf(ptr.pbuf + ptr.cnt, ptr.limit - ptr.cnt,
-	    " %d", ep->length);
-	mon_text_read_data(rp, &ptr, ep);
 
-	if (copy_to_user(buf, rp->printf_buf, ptr.cnt))
-		ptr.cnt = -EFAULT;
+	ret = mon_text_copy_to_user(rp, buf, nbytes);
 	mutex_unlock(&rp->printf_lock);
-	kmem_cache_free(rp->e_slab, ep);
-	return ptr.cnt;
+	return ret;
 }
 
 static struct mon_event_text *mon_text_read_wait(struct mon_reader_text *rp,