Date:   Tue, 10 Mar 2020 11:12:12 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [patch part-II V2 01/13] context_tracking: Ensure that the
 critical path cannot be instrumented


On 3/8/20 11:24 PM, Thomas Gleixner wrote:
> 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@...utronix.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(-)

Reviewed-by: Alexandre Chartre <alexandre.chartre@...cle.com>

alex.

> --- 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;
> 

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