From 7197e73a2e737d2932e393750637766d1e4be9a9 Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Wed, 12 Feb 2025 19:15:15 +0100
Subject: [PATCH] usb: cdc-acm: Check control transfer buffer size before
access
commit e563b01208f4d1f609bcab13333b6c0e24ce6a01 upstream.
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 ea2583529cd1
("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: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/usb/class/cdc-acm.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)
diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 38e7a9cf97656..27d0a8cb0fe52 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -360,7 +360,7 @@ 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;
@@ -387,9 +387,20 @@ 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);
--
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