fsnotify: Fix ordering of iput() and watched_objects decrement

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fsnotify: Fix ordering of iput() and watched_objects decrement Ensure the superblock is kept alive until we're done with iput(). Holding a reference to an inode is not allowed unless we ensure the superblock stays alive, which fsnotify does by keeping the watched_objects count elevated, so iput() must happen before the watched_objects decrement. This can lead to a UAF of something like sb->s_fs_info in tmpfs, but the UAF is hard to hit because race orderings that oops are more likely, thanks to the CHECK_DATA_CORRUPTION() block in generic_shutdown_super(). Also, ensure that fsnotify_put_sb_watched_objects() doesn't call fsnotify_sb_watched_objects() on a superblock that may have already been freed, which would cause a UAF read of sb->s_fsnotify_info.

References

• https://git.kernel.org/stable/c/45a8f8232a495221ed058191629f5c628f21601a
• https://git.kernel.org/stable/c/83af1cfa10d9aafdabd06b3655e07727f373b434
• https://git.kernel.org/stable/c/21d1b618b6b9da46c5116c640ac4b1cc8d40d63a
• https://project-zero.issues.chromium.org/issues/379667898