From: Masami Hiramatsu (Google) <[email protected]> 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. Reported-by: Sashiko <[email protected]> Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3 Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period") Cc: [email protected] Assisted-by: LLM Signed-off-by: Masami Hiramatsu (Google) <[email protected]> --- kernel/trace/fprobe.c | 26 ++++++++++++++++---------- 1 file changed, 16 insertions(+), 10 deletions(-) diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c index 9f2d98181779..da286619c5d8 100644 --- a/kernel/trace/fprobe.c +++ b/kernel/trace/fprobe.c @@ -47,6 +47,10 @@ static struct rhltable fprobe_ip_table; static DEFINE_MUTEX(fprobe_mutex); static struct fgraph_ops fprobe_graph_ops; +DEFINE_LOCK_GUARD_0(rcu_sched_notrace, + rcu_read_lock_sched_notrace(), + rcu_read_unlock_sched_notrace()) + static u32 fprobe_node_hashfn(const void *data, u32 len, u32 seed) { return hash_ptr(*(unsigned long **)data, 32); @@ -329,16 +333,14 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip, struct fprobe *fp; int bit; + if (!rcu_is_watching()) + return; + bit = ftrace_test_recursion_trylock(ip, parent_ip); if (bit < 0) return; - /* - * ftrace_test_recursion_trylock() disables preemption, but - * rhltable_lookup() checks whether rcu_read_lcok is held. - * So we take rcu_read_lock() here. - */ - rcu_read_lock(); + guard(rcu_sched_notrace)(); head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params); rhl_for_each_entry_rcu(node, pos, head, hlist) { @@ -353,7 +355,6 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip, else __fprobe_handler(ip, parent_ip, fp, fregs, NULL); } - rcu_read_unlock(); ftrace_test_recursion_unlock(bit); } NOKPROBE_SYMBOL(fprobe_ftrace_entry); @@ -567,10 +568,13 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops struct fprobe *fp; int used, ret; + if (!rcu_is_watching()) + return 0; + if (WARN_ON_ONCE(!fregs)) return 0; - guard(rcu)(); + guard(rcu_sched_notrace)(); head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params); reserved_words = 0; rhl_for_each_entry_rcu(node, pos, head, hlist) { @@ -665,13 +669,16 @@ static void fprobe_return(struct ftrace_graph_ret *trace, int size, curr; int size_words; + if (!rcu_is_watching()) + return; + fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size); if (WARN_ON_ONCE(!fgraph_data)) return; size_words = SIZE_IN_LONG(size); ret_ip = ftrace_regs_get_instruction_pointer(fregs); - preempt_disable_notrace(); + guard(rcu_sched_notrace)(); curr = 0; while (size_words > curr) { @@ -687,7 +694,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace, } curr += size; } - preempt_enable_notrace(); } NOKPROBE_SYMBOL(fprobe_return);
