CVE-2022-49834

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

nilfs2: fix use-after-free bug of ns_writer on remount

If a nilfs2 filesystem is downgraded to read-only due to metadata corruption on disk and is remounted read/write, or if emergency read-only remount is performed, detaching a log writer and synchronizing the filesystem can be done at the same time.

In these cases, use-after-free of the log writer (hereinafter nilfs->ns_writer) can happen as shown in the scenario below:

Task1 Task2

nilfs_construct_segment nilfs_segctor_sync init_wait init_waitqueue_entry add_wait_queue schedule nilfs_remount (R/W remount case) nilfs_attach_log_writer nilfs_detach_log_writer nilfs_segctor_destroy kfree finish_wait _raw_spin_lock_irqsave __raw_spin_lock_irqsave do_raw_spin_lock debug_spin_lock_before <-- use-after-free

While Task1 is sleeping, nilfs->ns_writer is freed by Task2. After Task1 waked up, Task1 accesses nilfs->ns_writer which is already freed. This scenario diagram is based on the Shigeru Yoshida's post [1].

This patch fixes the issue by not detaching nilfs->ns_writer on remount so that this UAF race doesn't happen. Along with this change, this patch also inserts a few necessary read-only checks with superblock instance where only the ns_writer pointer was used to check if the filesystem is read-only.

This vulnerability presents no operational impact on Red Hat Enterprise Linux as the NILFS2 filesystem driver is neither built nor enabled in Red Hat kernels. In upstream environments where this filesystem is active, exploitation requires local privileges to mount or remount a corrupted filesystem image, which restricts the attack scope. Because the vulnerable code is not included in supported Red Hat products, systems running default configurations are not exposed.