usb: gadget: f_fs: Fix epfile null pointer access after ep enable.

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_fs: Fix epfile null pointer access after ep enable. A race condition occurs when ffs_func_eps_enable() runs concurrently with ffs_data_reset(). The ffs_data_clear() called in ffs_data_reset() sets ffs->epfiles to NULL before resetting ffs->eps_count to 0, leading to a NULL pointer dereference when accessing epfile->ep in ffs_func_eps_enable() after successful usb_ep_enable(). The ffs->epfiles pointer is set to NULL in both ffs_data_clear() and ffs_data_close() functions, and its modification is protected by the spinlock ffs->eps_lock. And the whole ffs_func_eps_enable() function is also protected by ffs->eps_lock. Thus, add NULL pointer handling for ffs->epfiles in the ffs_func_eps_enable() function to fix issues

References

• https://git.kernel.org/stable/c/b00d2572c16e8e59e979960d3383c2ae9cebd195
• https://git.kernel.org/stable/c/1c0dbd240be3f87cac321b14e17979b7e9cb6a8f
• https://git.kernel.org/stable/c/9ec40fba7357df2d36f4c2e2f3b9b1a4fba0a272
• https://git.kernel.org/stable/c/c53e90563bc148e4e0ad09fe130ba2246d426ea6
• https://git.kernel.org/stable/c/fc1141a530dfc91f0ee19b7f422a2d24829584bc
• https://git.kernel.org/stable/c/d62b808d5c68a931ad0849a00a5e3be3dd7e0019
• https://git.kernel.org/stable/c/30880e9df27332403dd638a82c27921134b3630b
• https://git.kernel.org/stable/c/cfd6f1a7b42f62523c96d9703ef32b0dbc495ba4