usb: gadget: f_tcm: keep port count until LUN teardown completes

Overview

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

usb: gadget: f_tcm: keep port count until LUN teardown completes

tcm_usbg_drop_nexus() permits session removal once tpg_port_count reaches zero. However, usbg_port_unlink() currently decrements that count from the fabric_pre_unlink() callback, before core_dev_del_lun() waits for active se_lun references to drain.

If removal of the last LUN races a nexus removal, the latter can observe a zero port count and call target_remove_session(). This frees sess_cmd_map while an in-flight struct usbg_cmd, including its work item, can still be accessed.

Overlapping the last-LUN unlink with nexus removal reproduces this lifetime violation as a DEBUG_OBJECTS "free active" warning for usbg_cmd_work, followed by a target-core BUG/Oops.

The generic target-core unlink path has no callback after core_dev_del_lun() completes. Add an optional fabric_post_unlink() callback and use it for the f_tcm port count. The count now remains nonzero until core_dev_del_lun() has finished draining active LUN references, preventing nexus removal from freeing the session during command completion.

Affected

• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < c494c5562ca69b61a82f566e3b87a445d2c28929
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < c1f359d9a5efed458946063de65ddbeaacc4f165
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < 178f59a0bccd3f66cdfa5184310f31a58b7257c4
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < ad6f0375d2e93a1d8c015463e5e92dfcb26e311b
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < 2efbfd42441d3ef8137aff2d59e9835e1d5ae780
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < 85aa61fedcb4eb13f3dc5db73f6dc359f41f5d95
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < bbd6aa311a9f4dd17822c7557451458d3d2e980b
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < eaa96a8458f54d6cf0954242ab8b1df2a6fccafa
• Linux >= c52661d60f636d17e26ad834457db333bd1df494 < c39d0916da47d94909391876c9e5bd429ea7b1b9
• Linux 3.5

Remediation

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