fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

Overview

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

fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

unregister_fprobe() and unregister_fprobe_async() (used by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() and call_rcu()) to wait until in-flight fprobe handlers complete before freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such as in the idle loop or nohz_full extended quiescent states), standard RCU does not track preemption-disabled sections. Consequently, synchronize_rcu() does not wait for those executions, which can lead to a use-after-free if the fprobe is freed immediately after unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use rcu_dereference_all_check() (which checks rcu_read_lock_any_held()), holding preemption disabled via rcu_read_lock_sched_notrace() is fully valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(), fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path rcu_read_lock() and lockdep overhead while guaranteeing safe grace period synchronization.

Affected

• Linux >= 56b4cfcf1518245493c60fd39c56978f508f1816 < a85d0f00671b4d4527317c10ba018680f7d8d673
• Linux >= 657b594b2084b39a4bc6d8493aa2140cb00cea49 < dd4a0d4993633b45470abf6da4ea0ca563402245
• Linux >= 657b594b2084b39a4bc6d8493aa2140cb00cea49 < e0a6249190402a28f1e2925a81acf573157d55ef
• Linux a4f6a9005ed6cfd360ef2520430927f05f92ffb0
• Linux >= 6.18.34 < 6.18.56
• Linux >= 7.0.11 < 7.1
• Linux 7.1

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.