[PATCH 1/2] USB: cdc-acm: fix racy tty buffer accesses

From: Johan Hovold
Date: Wed Sep 29 2021 - 05:10:10 EST

A recent change that started reporting break events to the line
discipline caused the tty-buffer insertions to no longer be serialised
by inserting events also from the completion handler for the interrupt

Completion calls for distinct endpoints are not guaranteed to be
serialised. For example, in case a host-controller driver uses
bottom-half completion, the interrupt and bulk-in completion handlers
can end up running in parallel on two CPUs (high-and low-prio tasklets,
respectively) thereby breaking the tty layer's single producer

Fix this by holding the read lock also when inserting characters from
the bulk endpoint.

Fixes: 08dff274edda ("cdc-acm: fix BREAK rx code path adding necessary calls")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
drivers/usb/class/cdc-acm.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 4e2f1552f4b7..c7a1736720e7 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -475,11 +475,16 @@ static int acm_submit_read_urbs(struct acm *acm, gfp_t mem_flags)

static void acm_process_read_urb(struct acm *acm, struct urb *urb)
+ unsigned long flags;
if (!urb->actual_length)

+ spin_lock_irqsave(&acm->read_lock, flags);
tty_insert_flip_string(&acm->port, urb->transfer_buffer,
+ spin_unlock_irqrestore(&acm->read_lock, flags);