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usb: cdc-acm: Check control transfer buffer size before access

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If the first fragment is shorter than struct usb_cdc_notification, we can't
calculate an expected_size. Log an error and discard the notification
instead of reading lengths from memory outside the received data, which can
lead to memory corruption when the expected_size decreases between
fragments, causing `expected_size - acm->nb_index` to wrap.

This issue has been present since the beginning of git history; however,
it only leads to memory corruption since commit ea2583529c
("cdc-acm: reassemble fragmented notifications").

A mitigating factor is that acm_ctrl_irq() can only execute after userspace
has opened /dev/ttyACM*; but if ModemManager is running, ModemManager will
do that automatically depending on the USB device's vendor/product IDs and
its other interfaces.

Cc: stable <stable@kernel.org>
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Jann Horn 2025-02-12 19:15:15 +01:00 • committed by Greg Kroah-Hartman
parent c81d9fcd5b
commit e563b01208
1 changed files with 14 additions and 3 deletions
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17
drivers/usb/class/cdc-acm.c
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@ -371,7 +371,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
static void acm_ctrl_irq(struct urb *urb)
{
struct acm *acm = urb->context;
struct usb_cdc_notification *dr = urb->transfer_buffer;
struct usb_cdc_notification *dr;
unsigned int current_size = urb->actual_length;
unsigned int expected_size, copy_size, alloc_size;
int retval;
@ -398,9 +398,20 @@ static void acm_ctrl_irq(struct urb *urb)
usb_mark_last_busy(acm->dev);
if (acm->nb_index)
if (acm->nb_index == 0) {
/*
* The first chunk of a message must contain at least the
* notification header with the length field, otherwise we
* can't get an expected_size.
*/
if (current_size < sizeof(struct usb_cdc_notification)) {
dev_dbg(&acm->control->dev, "urb too short\n");
goto exit;
}
dr = urb->transfer_buffer;
} else {
dr = (struct usb_cdc_notification *)acm->notification_buffer;
}
/* size = notification-header + (optional) data */
expected_size = sizeof(struct usb_cdc_notification) +
le16_to_cpu(dr->wLength);