[patch part-II V2 01/13] context_tracking: Ensure that the critical path cannot be instrumented

Thomas Gleixner
2020-03-08T23:24:02Z

context tracking lacks a few protection mechanisms against instrumentation:

- While the core functions are marked NOKPROBE they lack protection
against function tracing which is required as the function entry/exit
points can be utilized by BPF.

- static functions invoked from the protected functions need to be marked
as well as they can be instrumented otherwise.

- using plain inline allows the compiler to emit traceable and probable
functions.

Fix this by adding the missing notrace/NOKPROBE annotations and converting
the plain inlines to __always_inline.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
include/linux/context_tracking.h | 14 +++++++-------
include/linux/context_tracking_state.h | 6 +++---
kernel/context_tracking.c | 9 +++++----
3 files changed, 15 insertions(+), 14 deletions(-)

--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -20,32 +20,32 @@ extern void context_tracking_exit(enum c
extern void context_tracking_user_enter(void);
extern void context_tracking_user_exit(void);

-static inline void user_enter(void)
+static __always_inline void user_enter(void)
{
if (context_tracking_enabled())
context_tracking_enter(CONTEXT_USER);

}
-static inline void user_exit(void)
+static __always_inline void user_exit(void)
{
if (context_tracking_enabled())
context_tracking_exit(CONTEXT_USER);
}

/* Called with interrupts disabled. */
-static inline void user_enter_irqoff(void)
+static __always_inline void user_enter_irqoff(void)
{
if (context_tracking_enabled())
__context_tracking_enter(CONTEXT_USER);

}
-static inline void user_exit_irqoff(void)
+static __always_inline void user_exit_irqoff(void)
{
if (context_tracking_enabled())
__context_tracking_exit(CONTEXT_USER);
}

-static inline enum ctx_state exception_enter(void)
+static __always_inline enum ctx_state exception_enter(void)
{
enum ctx_state prev_ctx;

@@ -59,7 +59,7 @@ static inline enum ctx_state exception_e
return prev_ctx;
}

-static inline void exception_exit(enum ctx_state prev_ctx)
+static __always_inline void exception_exit(enum ctx_state prev_ctx)
{
if (context_tracking_enabled()) {
if (prev_ctx != CONTEXT_KERNEL)
@@ -75,7 +75,7 @@ static inline void exception_exit(enum c
* is enabled. If context tracking is disabled, returns
* CONTEXT_DISABLED. This should be used primarily for debugging.
*/
-static inline enum ctx_state ct_state(void)
+static __always_inline enum ctx_state ct_state(void)
{
return context_tracking_enabled() ?
this_cpu_read(context_tracking.state) : CONTEXT_DISABLED;
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -26,12 +26,12 @@ struct context_tracking {
extern struct static_key_false context_tracking_key;
DECLARE_PER_CPU(struct context_tracking, context_tracking);

-static inline bool context_tracking_enabled(void)
+static __always_inline bool context_tracking_enabled(void)
{
return static_branch_unlikely(&context_tracking_key);
}

-static inline bool context_tracking_enabled_cpu(int cpu)
+static __always_inline bool context_tracking_enabled_cpu(int cpu)
{
return context_tracking_enabled() && per_cpu(context_tracking.active, cpu);
}
@@ -41,7 +41,7 @@ static inline bool context_tracking_enab
return context_tracking_enabled() && __this_cpu_read(context_tracking.active);
}

-static inline bool context_tracking_in_user(void)
+static __always_inline bool context_tracking_in_user(void)
{
return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
}
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -31,7 +31,7 @@ EXPORT_SYMBOL_GPL(context_tracking_key);
DEFINE_PER_CPU(struct context_tracking, context_tracking);
EXPORT_SYMBOL_GPL(context_tracking);

-static bool context_tracking_recursion_enter(void)
+static notrace bool context_tracking_recursion_enter(void)
{
int recursion;

@@ -44,8 +44,9 @@ static bool context_tracking_recursion_e

return false;
}
+NOKPROBE_SYMBOL(context_tracking_recursion_enter);

-static void context_tracking_recursion_exit(void)
+static __always_inline void context_tracking_recursion_exit(void)
{
__this_cpu_dec(context_tracking.recursion);
}
@@ -59,7 +60,7 @@ static void context_tracking_recursion_e
* instructions to execute won't use any RCU read side critical section
* because this function sets RCU in extended quiescent state.
*/
-void __context_tracking_enter(enum ctx_state state)
+void notrace __context_tracking_enter(enum ctx_state state)
{
/* Kernel threads aren't supposed to go to userspace */
WARN_ON_ONCE(!current->mm);
@@ -142,7 +143,7 @@ NOKPROBE_SYMBOL(context_tracking_user_en
* This call supports re-entrancy. This way it can be called from any exception
* handler without needing to know if we came from userspace or not.
*/
-void __context_tracking_exit(enum ctx_state state)
+void notrace __context_tracking_exit(enum ctx_state state)
{
if (!context_tracking_recursion_enter())
return;