spi: spidev: fix lock inversion between spi_lock and buf_lock

Overview

In the Linux kernel, the following vulnerability has been resolved:

spi: spidev: fix lock inversion between spi_lock and buf_lock

The spidev driver previously used two mutexes, spi_lock and buf_lock, but acquired them in different orders depending on the code path:

write()/read(): buf_lock -> spi_lock ioctl(): spi_lock -> buf_lock

This AB-BA locking pattern triggers lockdep warnings and can cause real deadlocks:

WARNING: possible circular locking dependency detected spidev_ioctl() -> mutex_lock(&spidev->buf_lock) spidev_sync_write() -> mutex_lock(&spidev->spi_lock) *** DEADLOCK ***

The issue is reproducible with a simple userspace program that performs write() and SPI_IOC_WR_MAX_SPEED_HZ ioctl() calls from separate threads on the same spidev file descriptor.

Fix this by simplifying the locking model and removing the lock inversion entirely. spidev_sync() no longer performs any locking, and all callers serialize access using spi_lock.

buf_lock is removed since its functionality is fully covered by spi_lock, eliminating the possibility of lock ordering issues.

This removes the lock inversion and prevents deadlocks without changing userspace ABI or behaviour.

Affected

• Linux >= a720416d94634068951773cb9e9d6f1b73769e5b < fabfed1afe273717ea33b8aee46b767360edbb80
• Linux >= a720416d94634068951773cb9e9d6f1b73769e5b < f8431b8672231d378b03176fe74c95adfd3522cf
• Linux >= a720416d94634068951773cb9e9d6f1b73769e5b < e341e18215030af2136836b78508e0d798916df7
• Linux >= a720416d94634068951773cb9e9d6f1b73769e5b < 41ccfac7d302968a4f32b5f7b012d066c5f5cdf8
• Linux >= a720416d94634068951773cb9e9d6f1b73769e5b < 40534d19ed2afb880ecf202dab26a8e7a5808d16
• Linux 6.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.