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

From: Stephen Brennan <[email-masked]>
Date: Mon, 29 Apr 2024 10:47:26 -0700

Masami Hiramatsu (Google) <[email-masked]> writes:
> Hi Stephen,
>
> On Fri, 26 Apr 2024 15:58:34 -0700
> Stephen Brennan <[email-masked]> 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.
>
> Hmm, indeed.
>
>> 
>> 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.
>
> OK, the patch looks good to me.
>
> Acked-by: Masami Hiramatsu (Google) <[email-masked]>
>
> Thanks!

Hi Masami,

Thank you! Sadly I took a second look at the patch and noticed I forgot
to remove the existing declarations of ftrace_is_dead() from
kernel/trace/trace.h. I've sent v2 in reply to v1 in order to correct
that. I'm sorry for the churn.

Thanks,
Stephen

>> 
>> Signed-off-by: Stephen Brennan <[email-masked]>
>> ---
>> 
>> Apologies for the wide net cast here. I recognize that a change like this
>> may need to be split up and go through arch-specific trees. I hoped to get
>> feedback on the patch itself. If it's satisfactory and the architecture
>> maintainers prefer it split out, I'm glad to do it. Thanks!
>> 
>>  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 ++
>>  8 files changed, 23 insertions(+)
>> 
>> 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)
>>  {
>> -- 
>> 2.39.3
>> 
>
>
> -- 
> Masami Hiramatsu (Google) <[email-masked]>