scsi: target: Fix recursive locking in __configfs_open_file()

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

scsi: target: Fix recursive locking in __configfs_open_file()

In flush_write_buffer, &p->frag_sem is acquired and then the loaded store function is called, which, here, is target_core_item_dbroot_store(). This function called filp_open(), following which these functions were called (in reverse order), according to the call trace:

down_read __configfs_open_file do_dentry_open vfs_open do_open path_openat do_filp_open file_open_name filp_open target_core_item_dbroot_store flush_write_buffer configfs_write_iter

target_core_item_dbroot_store() tries to validate the new file path by trying to open the file path provided to it; however, in this case, the bug report shows:

db_root: not a directory: /sys/kernel/config/target/dbroot

indicating that the same configfs file was tried to be opened, on which it is currently working on. Thus, it is trying to acquire frag_sem semaphore of the same file of which it already holds the semaphore obtained in flush_write_buffer(), leading to acquiring the semaphore in a nested manner and a possibility of recursive locking.

Fix this by modifying target_core_item_dbroot_store() to use kern_path() instead of filp_open() to avoid opening the file using filesystem-specific function __configfs_open_file(), and further modifying it to make this fix compatible.