writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

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

writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

When a container exits, the following BUG_ON() is occasionally triggered:

==================================================================
VFS: Busy inodes after unmount of sdb (ext4)
------------[ cut here ]------------
kernel BUG at fs/super.c:695!
CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1
pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : generic_shutdown_super+0xf0/0x100
lr : generic_shutdown_super+0xf0/0x100
Call trace:
generic_shutdown_super+0xf0/0x100
kill_block_super+0x20/0x48
ext4_kill_sb+0x28/0x60
deactivate_locked_super+0x54/0x130
deactivate_super+0x84/0xa0
cleanup_mnt+0xa4/0x140
__cleanup_mnt+0x18/0x28
task_work_run+0x78/0xe0
do_notify_resume+0x204/0x240
==================================================================

The root cause is a race between cgroup_writeback_umount() and
inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between
inode_prepare_wbs_switch() returning true and the subsequent
wb_queue_isw() call. Following is the process that triggers the issue:

CPU A (umount) | CPU B (writeback)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
inode_switch_wbs/cleanup_offline_cgwb
atomic_inc(&isw_nr_in_flight)
inode_prepare_wbs_switch
-> passes SB_ACTIVE check
__iget(inode)
generic_shutdown_super
sb->s_flags &= ~SB_ACTIVE
cgroup_writeback_umount(sb)
smp_mb()
atomic_read(&isw_nr_in_flight)
rcu_barrier()
-> no pending RCU callbacks
flush_workqueue(isw_wq)
-> nothing queued, returns
evict_inodes(sb)
-> Inode skipped as isw still holds a ref.
sop->put_super(sb)
/* destroys percpu counters */
-> VFS: Busy inodes after unmount!
wb_queue_isw()
queue_work(isw_wq, ...)
/* later in work function */
inode_switch_wbs_work_fn
process_inode_switch_wbs
iput() -> evict
percpu_counter_dec() // UAF!

Fix this by extending the RCU read-side critical section in
inode_switch_wbs() and cleanup_offline_cgwb() to cover from
inode_prepare_wbs_switch() through wb_queue_isw(). Since there is
no sleep in this window, rcu_read_lock() can be used. Then add a
synchronize_rcu() in cgroup_writeback_umount() before the existing
rcu_barrier(), so that all in-flight switchers that have passed the
SB_ACTIVE check have completed queue_work() before flush_workqueue()
is called.

The existing rcu_barrier() is intentionally retained so this fix can
be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that
still queue switches via queue_rcu_work(). It is a no-op on current
mainline (since commit e1b849cfa6b6 ("writeback: Avoid contention on
wb->list_lock when switching inodes")) and is removed in a follow-up
patch.