USB: cdc-acm: rework notification_buffer resizing

Clang static analysis reports this error

cdc-acm.c:409:3: warning: Use of memory after it is freed
        acm_process_notification(acm, (unsigned char *)dr);

There are three problems, the first one is that dr is not reset

The variable dr is set with

if (acm->nb_index)
	dr = (struct usb_cdc_notification *)acm->notification_buffer;

But if the notification_buffer is too small it is resized with

		if (acm->nb_size) {
			kfree(acm->notification_buffer);
			acm->nb_size = 0;
		}
		alloc_size = roundup_pow_of_two(expected_size);
		/*
		 * kmalloc ensures a valid notification_buffer after a
		 * use of kfree in case the previous allocation was too
		 * small. Final freeing is done on disconnect.
		 */
		acm->notification_buffer =
			kmalloc(alloc_size, GFP_ATOMIC);

dr should point to the new acm->notification_buffer.

The second problem is any data in the notification_buffer is lost
when the pointer is freed.  In the normal case, the current data
is accumulated in the notification_buffer here.

	memcpy(&acm->notification_buffer[acm->nb_index],
	       urb->transfer_buffer, copy_size);

When a resize happens, anything before
notification_buffer[acm->nb_index] is garbage.

The third problem is the acm->nb_index is not reset on a
resizing buffer error.

So switch resizing to using krealloc and reassign dr and
reset nb_index.

Fixes: ea2583529cd1 ("cdc-acm: reassemble fragmented notifications")
Signed-off-by: Tom Rix <trix@redhat.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20200801152154.20683-1-trix@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 991786876dbb3cb598c44c1ccb0c250d55c16e08..7f6f3ab5b8a6770060cae81445aecd9e88d1cc48 100644 (file)
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -378,21 +378,19 @@ static void acm_ctrl_irq(struct urb *urb)
        if (current_size < expected_size) {
                /* notification is transmitted fragmented, reassemble */
                if (acm->nb_size < expected_size) {
-                       if (acm->nb_size) {
-                               kfree(acm->notification_buffer);
-                               acm->nb_size = 0;
-                       }
+                       u8 *new_buffer;
                        alloc_size = roundup_pow_of_two(expected_size);
-                       /*
-                        * kmalloc ensures a valid notification_buffer after a
-                        * use of kfree in case the previous allocation was too
-                        * small. Final freeing is done on disconnect.
-                        */
-                       acm->notification_buffer =
-                               kmalloc(alloc_size, GFP_ATOMIC);
-                       if (!acm->notification_buffer)
+                       /* Final freeing is done on disconnect. */
+                       new_buffer = krealloc(acm->notification_buffer,
+                                             alloc_size, GFP_ATOMIC);
+                       if (!new_buffer) {
+                               acm->nb_index = 0;
                                goto exit;
+                       }
+
+                       acm->notification_buffer = new_buffer;
                        acm->nb_size = alloc_size;
+                       dr = (struct usb_cdc_notification *)acm->notification_buffer;
                }
 
                copy_size = min(current_size,