2021-02-02 07:26:46

by Alexey Kardashevskiy

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

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 <[email protected]>
---

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

2021-02-02 10:13:15

by Peter Zijlstra

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

On Tue, Feb 02, 2021 at 06:23:26PM +1100, Alexey Kardashevskiy wrote:
> 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.

Agreed, a reload got added and it doesn't check the value again.

> Fixes: d25e37d89dd2f ("tracepoint: Optimize using static_call()")
> Signed-off-by: Alexey Kardashevskiy <[email protected]>

> it_func_ptr = \
> rcu_dereference_raw((&__tracepoint_##_name)->funcs); \

> + 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; \


Acked-by: Peter Zijlstra (Intel) <[email protected]>