In the Linux kernel, the following vulnerability has been resolved: eventfs: Hold eventfs_mutex...

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

eventfs: Hold eventfs_mutex and SRCU when remount walks events

Commit 340f0c7067a9 ("eventfs: Update all the eventfs_inodes from the
events descriptor") had eventfs_set_attrs() recurse through ei->children
on remount. The walk only holds the rcu_read_lock() taken by
tracefs_apply_options() over tracefs_inodes, which is wrong:

• list_for_each_entry over ei->children races with the list_del_rcu()
  in eventfs_remove_rec() -- LIST_POISON1 deref, same shape as
  d2603279c7d6.
• eventfs_inodes are freed via call_srcu(&eventfs_srcu, ...).
  rcu_read_lock() does not extend an SRCU grace period, so ti->private
  can be reclaimed under the walk.
• The writes to ei->attr race with eventfs_set_attr(), which holds
  eventfs_mutex.

Reproducer:

while :; do mount -o remount,uid=$((RANDOM%1000)) /sys/kernel/tracing; done &
while :; do
echo "p:kp submit_bio" > /sys/kernel/tracing/kprobe_events
echo > /sys/kernel/tracing/kprobe_events
done

Wrap the events portion of tracefs_apply_options() in
eventfs_remount_lock()/_unlock() that take eventfs_mutex and
srcu_read_lock(&eventfs_srcu). eventfs_set_attrs() doesn't sleep so the
nested rcu_read_lock() is fine; lockdep_assert_held() pins the contract.

Comment in tracefs_drop_inode() said "RCU cycle" -- it is SRCU.