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From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: Steven Rostedt <rostedt@goodmis.org>
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Subject: Re: [PATCH v2] kprobe/ftrace: bail out if ftrace was killed
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Date: Tue, 30 Apr 2024 16:01:43 -0700
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Steven Rostedt <rostedt@goodmis.org> writes:
> On Mon, 29 Apr 2024 10:47:18 -0700
> Stephen Brennan <stephen.s.brennan@oracle.com> 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@oracle.com>
>> ---
>> 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.

Thanks, that does make a lot more sense. It's faster, and doesn't
involve exporting that function. I'll go ahead and use this approach.

Thanks,
Stephen