media: rainshadow-cec: fix TOCTOU race condition in rain_interrupt()

media: rainshadow-cec: fix TOCTOU race condition in rain_interrupt()

In the interrupt handler rain_interrupt(), the buffer full check on
rain->buf_len is performed before acquiring rain->buf_lock. This
creates a Time-of-Check to Time-of-Use (TOCTOU) race condition, as
rain->buf_len is concurrently accessed and modified in the work
handler rain_irq_work_handler() under the same lock.

Multiple interrupt invocations can race, with each reading buf_len
before it becomes full and then proceeding. This can lead to both
interrupts attempting to write to the buffer, incrementing buf_len
beyond its capacity (DATA_SIZE) and causing a buffer overflow.

Fix this bug by moving the spin_lock() to before the buffer full
check. This ensures that the check and the subsequent buffer modification
are performed atomically, preventing the race condition. An corresponding
spin_unlock() is added to the overflow path to correctly release the
lock.

This possible bug was found by an experimental static analysis tool
developed by our team.

Fixes: 0f314f6c2e77 ("[media] rainshadow-cec: new RainShadow Tech HDMI CEC driver")
Cc: stable@vger.kernel.org
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>