Re: [PATCH v2] kprobe/ftrace: bail out if ftrace was killed

[Steven Rostedt]

On Mon, 29 Apr 2024 10:47:18 -0700
Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> wrote:

> If an error happens in ftrace, ftrace_kill() will prevent disarming
> kprobes. Eventually, the ftrace_ops associated with the kprobes will be
> freed, yet the kprobes will still be active, and when triggered, they
> will use the freed memory, likely resulting in a page fault and panic.
> 
> This behavior can be reproduced quite easily, by creating a kprobe and
> then triggering a ftrace_kill(). For simplicity, we can simulate an
> ftrace error with a kernel module like [1]:
> 
> [1]: https://github.com/brenns10/kernel_stuff/tree/master/ftrace_killer
> 
>   sudo perf probe --add commit_creds
>   sudo perf trace -e probe:commit_creds
>   # In another terminal
>   make
>   sudo insmod ftrace_killer.ko  # calls ftrace_kill(), simulating bug
>   # Back to perf terminal
>   # ctrl-c
>   sudo perf probe --del commit_creds
> 
> After a short period, a page fault and panic would occur as the kprobe
> continues to execute and uses the freed ftrace_ops. While ftrace_kill()
> is supposed to be used only in extreme circumstances, it is invoked in
> FTRACE_WARN_ON() and so there are many places where an unexpected bug
> could be triggered, yet the system may continue operating, possibly
> without the administrator noticing. If ftrace_kill() does not panic the
> system, then we should do everything we can to continue operating,
> rather than leave a ticking time bomb.
> 
> Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
> ---
> Difference from v1: removed both existing declarations of ftrace_is_dead()
> from kernel/trace/trace.h.
> 
>  arch/csky/kernel/probes/ftrace.c     | 3 +++
>  arch/loongarch/kernel/ftrace_dyn.c   | 3 +++
>  arch/parisc/kernel/ftrace.c          | 3 +++
>  arch/powerpc/kernel/kprobes-ftrace.c | 3 +++
>  arch/riscv/kernel/probes/ftrace.c    | 3 +++
>  arch/s390/kernel/ftrace.c            | 3 +++
>  arch/x86/kernel/kprobes/ftrace.c     | 3 +++
>  include/linux/ftrace.h               | 2 ++
>  kernel/trace/trace.h                 | 2 --
>  9 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> index 834cffcfbce3..3931bf9f707b 100644
> --- a/arch/csky/kernel/probes/ftrace.c
> +++ b/arch/csky/kernel/probes/ftrace.c
> @@ -12,6 +12,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe_ctlblk *kcb;
>  	struct pt_regs *regs;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/loongarch/kernel/ftrace_dyn.c b/arch/loongarch/kernel/ftrace_dyn.c
> index 73858c9029cc..82c952cb5be0 100644
> --- a/arch/loongarch/kernel/ftrace_dyn.c
> +++ b/arch/loongarch/kernel/ftrace_dyn.c
> @@ -287,6 +287,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> index 621a4b386ae4..3660834f54c3 100644
> --- a/arch/parisc/kernel/ftrace.c
> +++ b/arch/parisc/kernel/ftrace.c
> @@ -206,6 +206,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe *p;
>  	int bit;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> index 072ebe7f290b..85eb55aa1457 100644
> --- a/arch/powerpc/kernel/kprobes-ftrace.c
> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
> @@ -21,6 +21,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>  	struct pt_regs *regs;
>  	int bit;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(nip, parent_nip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/riscv/kernel/probes/ftrace.c b/arch/riscv/kernel/probes/ftrace.c
> index 7142ec42e889..8814fbe4c888 100644
> --- a/arch/riscv/kernel/probes/ftrace.c
> +++ b/arch/riscv/kernel/probes/ftrace.c
> @@ -11,6 +11,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe_ctlblk *kcb;
>  	int bit;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
> index c46381ea04ec..ccbe8ccf945b 100644
> --- a/arch/s390/kernel/ftrace.c
> +++ b/arch/s390/kernel/ftrace.c
> @@ -296,6 +296,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe *p;
>  	int bit;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
> index dd2ec14adb77..c73f9ab7ff50 100644
> --- a/arch/x86/kernel/kprobes/ftrace.c
> +++ b/arch/x86/kernel/kprobes/ftrace.c
> @@ -21,6 +21,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  	struct kprobe_ctlblk *kcb;
>  	int bit;
>  
> +	if (unlikely(ftrace_is_dead()))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
>  		return;
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 54d53f345d14..ba83e99c1fbe 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -399,6 +399,7 @@ int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *a
>  #define register_ftrace_function(ops) ({ 0; })
>  #define unregister_ftrace_function(ops) ({ 0; })
>  static inline void ftrace_kill(void) { }
> +static inline int ftrace_is_dead(void) { return 0; }
>  static inline void ftrace_free_init_mem(void) { }
>  static inline void ftrace_free_mem(struct module *mod, void *start, void *end) { }
>  static inline int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs)
> @@ -914,6 +915,7 @@ static inline bool is_ftrace_trampoline(unsigned long addr)
>  
>  /* totally disable ftrace - can not re-enable after this */
>  void ftrace_kill(void);
> +int ftrace_is_dead(void);
>  
>  static inline void tracer_disable(void)
>  {
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 64450615ca0c..70a37ee41813 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1026,7 +1026,6 @@ static inline int ftrace_trace_task(struct trace_array *tr)
>  	return this_cpu_read(tr->array_buffer.data->ftrace_ignore_pid) !=
>  		FTRACE_PID_IGNORE;
>  }
> -extern int ftrace_is_dead(void);

Honestly I rather not expose this function outside of the tracing
infrastructure. Instead, we should have a kprobe_ftrace_kill() function,
and have ftrace_kill() call that.

Then kprobe_ftrace_kill() can set its own variable that is exposed to all
these functions and they can test that instead of adding the extra overhead
in the fast path of a function call to ftrace_is_dead()

extern bool kprobes_ftrace_disabled __read_mostly;

void kprobe_ftrace_kill(void)
{
	kprobes_ftrace_disabled = true;
}

And you can then replace all these with:

	if (kprobes_ftrace_disabled)
		return;

Which is faster.

-- Steve

>  int ftrace_create_function_files(struct trace_array *tr,
>  				 struct dentry *parent);
>  void ftrace_destroy_function_files(struct trace_array *tr);
> @@ -1046,7 +1045,6 @@ static inline int ftrace_trace_task(struct trace_array *tr)
>  {
>  	return 1;
>  }
> -static inline int ftrace_is_dead(void) { return 0; }
>  static inline int
>  ftrace_create_function_files(struct trace_array *tr,
>  			     struct dentry *parent)