Subject: [PATCH 21/28] cfi: Use RCU while invoking __module_address().
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Wed, 8 Jan 2025 10:04:50 +0100

__module_address() can be invoked within a RCU section, there is no
requirement to have preemption disabled.

The _notrace() variant was introduced in commit 14c4c8e41511a ("cfi: Use
rcu_read_{un}lock_sched_notrace"). The recursive case where
__cfi_slowpath_diag() could end up calling itself is no longer present,
as all that logic is gone since commit 89245600941e ("cfi: Switch to
-fsanitize=kcfi").
Sami Tolvanen said that KCFI checks don't perform function calls.

Elliot Berman verified it with
| modprobe -a dummy_stm stm_ftrace stm_p_basic
| mkdir -p /sys/kernel/config/stp-policy/dummy_stm.0.my-policy/default
| echo function > /sys/kernel/tracing/current_tracer
| echo 1 > /sys/kernel/tracing/tracing_on
| echo dummy_stm.0 > /sys/class/stm_source/ftrace/stm_source_link

Replace the rcu_read_lock_sched_notrace() section around
__module_address() with RCU.

Cc: Elliot Berman <[email-masked]>
Cc: Kees Cook <[email-masked]>
Cc: Nathan Chancellor <[email-masked]>
Cc: Sami Tolvanen <[email-masked]>
Cc: Steven Rostedt <[email-masked]>
Cc: [email-masked]
Tested-by: Elliot Berman <[email-masked]>  # sm8650-qrd
Link: https://lore.kernel.org/all/[email-masked]
Acked-by: Peter Zijlstra (Intel) <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/cfi.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- a/kernel/cfi.c
+++ b/kernel/cfi.c
@@ -71,14 +71,11 @@ static bool is_module_cfi_trap(unsigned
 	struct module *mod;
 	bool found = false;
 
-	rcu_read_lock_sched_notrace();
-
+	guard(rcu)();
 	mod = __module_address(addr);
 	if (mod)
 		found = is_trap(addr, mod->kcfi_traps, mod->kcfi_traps_end);
 
-	rcu_read_unlock_sched_notrace();
-
 	return found;
 }
 #else /* CONFIG_MODULES */