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From:   Elliot Berman <quic_eberman@quicinc.com>
To:     Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>
CC:     Elliot Berman <quic_eberman@quicinc.com>,
        <linux-kernel@vger.kernel.org>,
        Jinlong Mao <jinlmao@codeaurora.org>
Subject: [PATCH 1/1] cfi: Use rcu_read_{un}lock_sched_notrace
Date:   Wed, 11 Aug 2021 08:59:14 -0700
Message-ID: <20210811155914.19550-1-quic_eberman@quicinc.com>
MIME-Version: 1.0
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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>
---
 kernel/cfi.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/cfi.c b/kernel/cfi.c
index e17a56639766..9594cfd1cf2c 100644
--- a/kernel/cfi.c
+++ b/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;
 }
-- 
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