[PATCH kernel] tracepoint: Fix race between tracing and removing tracepoint

[Alexey Kardashevskiy <aik@ozlabs.ru>]

When executing a tracepoint, the tracepoint's func is dereferenced twice -
in __DO_TRACE() (where the returned pointer is checked) and later on in
__traceiter_##_name where the returned pointer is dereferenced without
checking which leads to races against tracepoint_removal_sync() and
crashes.

This adds a check before referencing the pointer in tracepoint_ptr_deref.

Fixes: d25e37d89dd2f ("tracepoint: Optimize using static_call()")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

This is in reply to https://lkml.org/lkml/2021/2/1/868

Feel free to change the commit log. Thanks!

Fixing it properly is rather scary :)
I tried passing it_func_ptr to it_func but this change triggered way too
many prototypes changes such as __bpf_trace_##call().

---
 include/linux/tracepoint.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0f21617f1a66..966ed8980327 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -307,11 +307,13 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 									\
 		it_func_ptr =						\
 			rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
-		do {							\
-			it_func = (it_func_ptr)->func;			\
-			__data = (it_func_ptr)->data;			\
-			((void(*)(void *, proto))(it_func))(__data, args); \
-		} while ((++it_func_ptr)->func);			\
+		if (it_func_ptr) {					\
+			do {						\
+				it_func = (it_func_ptr)->func;		\
+				__data = (it_func_ptr)->data;		\
+				((void(*)(void *, proto))(it_func))(__data, args); \
+			} while ((++it_func_ptr)->func);		\
+		}							\
 		return 0;						\
 	}								\
 	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);
-- 
2.17.1