cfi: Use rcu_read_{un}lock_sched_notrace

If rcu_read_lock_sched tracing is enabled, the tracing subsystem can
perform a jump which needs to be checked by CFI. For example, stm_ftrace
source is enabled as a module and hooks into enabled ftrace events. This
can cause an recursive loop where find_shadow_check_fn ->
rcu_read_lock_sched -> (call to stm_ftrace generates cfi slowpath) ->
find_shadow_check_fn -> rcu_read_lock_sched -> ...

To avoid the recursion, either the ftrace codes needs to be marked with
__no_cfi or CFI should not trace. Use the "_notrace" in CFI to avoid
tracing so that CFI can guard ftrace.
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Cc: stable@vger.kernel.org
Fixes: cf68fffb ("add support for Clang CFI")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210811155914.19550-1-quic_eberman@quicinc.com

kernel/cfi.c

@@ -248,9 +248,9 @@ static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
 {
 	cfi_check_fn fn;
 
-	rcu_read_lock_sched();
+	rcu_read_lock_sched_notrace();
 	fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
-	rcu_read_unlock_sched();
+	rcu_read_unlock_sched_notrace();
 
 	return fn;
 }
@@ -269,11 +269,11 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
 	cfi_check_fn fn = NULL;
 	struct module *mod;
 
-	rcu_read_lock_sched();
+	rcu_read_lock_sched_notrace();
 	mod = __module_address(ptr);
 	if (mod)
 		fn = mod->cfi_check;
-	rcu_read_unlock_sched();
+	rcu_read_unlock_sched_notrace();
 
 	return fn;
 }