Date:   Mon, 18 Dec 2017 18:11:12 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ast@...nel.org, kernel-team@...com, daniel@...earbox.net,
        linux-btrfs@...r.kernel.org, darrick.wong@...cle.com,
        Josef Bacik <jbacik@...com>
Subject: Re: [PATCH v10 1/5] add infrastructure for tagging functions as
 error injectable

On Fri, 15 Dec 2017 14:12:52 -0500
Josef Bacik <josef@...icpanda.com> wrote:

> From: Josef Bacik <jbacik@...com>
> 
> Using BPF we can override kprob'ed functions and return arbitrary
> values.  Obviously this can be a bit unsafe, so make this feature opt-in
> for functions.  Simply tag a function with KPROBE_ERROR_INJECT_SYMBOL in
> order to give BPF access to that function for error injection purposes.
> 

NAK. I'm very confused. What the reason to add this feature is implemented
in kernel/kprobes.c? It is seemed within an usual "usage" of kprobes.
I recommend you to implement this somewhere else... like
kernel/error_injection.c, or kernel/module.c.

More precisely list up the reasons why,

 - This is just for providing an API to check the address within an 
  address-range list inside kmodule (not related to kprobes).
 - There is no check in kprobes to modified address by using the API.
  (yes, that will cause a big overhead...)
 - This can mislead user NOT to change the instruction pointer from
  the kprobes except for that list.
 - If user intends to insert a piece of code (like livepatch) in a
  function, they do NOT think it is an "error injection".
 - Or if they find this API, and "what?? I can not change instruction
  pointer by kprobes? but I can." and report it a bug on lkml...

So, I don't like to see this in kprobes.c. It is better to make another
layer to do this.

Thank you,

> Signed-off-by: Josef Bacik <jbacik@...com>
> Acked-by: Ingo Molnar <mingo@...nel.org>
> ---
>  include/asm-generic/vmlinux.lds.h |  10 +++
>  include/linux/bpf.h               |  11 +++
>  include/linux/kprobes.h           |   1 +
>  include/linux/module.h            |   5 ++
>  kernel/kprobes.c                  | 163 ++++++++++++++++++++++++++++++++++++++
>  kernel/module.c                   |   6 +-
>  6 files changed, 195 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index ee8b707d9fa9..a2e8582d094a 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -136,6 +136,15 @@
>  #define KPROBE_BLACKLIST()
>  #endif
>  
> +#ifdef CONFIG_BPF_KPROBE_OVERRIDE
> +#define ERROR_INJECT_LIST()	. = ALIGN(8);						\
> +				VMLINUX_SYMBOL(__start_kprobe_error_inject_list) = .;	\
> +				KEEP(*(_kprobe_error_inject_list))			\
> +				VMLINUX_SYMBOL(__stop_kprobe_error_inject_list) = .;
> +#else
> +#define ERROR_INJECT_LIST()
> +#endif
> +
>  #ifdef CONFIG_EVENT_TRACING
>  #define FTRACE_EVENTS()	. = ALIGN(8);					\
>  			VMLINUX_SYMBOL(__start_ftrace_events) = .;	\
> @@ -564,6 +573,7 @@
>  	FTRACE_EVENTS()							\
>  	TRACE_SYSCALLS()						\
>  	KPROBE_BLACKLIST()						\
> +	ERROR_INJECT_LIST()						\
>  	MEM_DISCARD(init.rodata)					\
>  	CLK_OF_TABLES()							\
>  	RESERVEDMEM_OF_TABLES()						\
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index e55e4255a210..7f4d2a953173 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -576,4 +576,15 @@ extern const struct bpf_func_proto bpf_sock_map_update_proto;
>  void bpf_user_rnd_init_once(void);
>  u64 bpf_user_rnd_u32(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
>  
> +#if defined(__KERNEL__) && !defined(__ASSEMBLY__)
> +#ifdef CONFIG_BPF_KPROBE_OVERRIDE
> +#define BPF_ALLOW_ERROR_INJECTION(fname)				\
> +static unsigned long __used						\
> +	__attribute__((__section__("_kprobe_error_inject_list")))	\
> +	_eil_addr_##fname = (unsigned long)fname;
> +#else
> +#define BPF_ALLOW_ERROR_INJECTION(fname)
> +#endif
> +#endif
> +
>  #endif /* _LINUX_BPF_H */
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index 9440a2fc8893..963fd364f3d6 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -271,6 +271,7 @@ extern bool arch_kprobe_on_func_entry(unsigned long offset);
>  extern bool kprobe_on_func_entry(kprobe_opcode_t *addr, const char *sym, unsigned long offset);
>  
>  extern bool within_kprobe_blacklist(unsigned long addr);
> +extern bool within_kprobe_error_injection_list(unsigned long addr);
>  
>  struct kprobe_insn_cache {
>  	struct mutex mutex;
> diff --git a/include/linux/module.h b/include/linux/module.h
> index c69b49abe877..548fa09fa806 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -475,6 +475,11 @@ struct module {
>  	ctor_fn_t *ctors;
>  	unsigned int num_ctors;
>  #endif
> +
> +#ifdef CONFIG_BPF_KPROBE_OVERRIDE
> +	unsigned int num_kprobe_ei_funcs;
> +	unsigned long *kprobe_ei_funcs;
> +#endif
>  } ____cacheline_aligned __randomize_layout;
>  #ifndef MODULE_ARCH_INIT
>  #define MODULE_ARCH_INIT {}
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index da2ccf142358..b4aab48ad258 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -83,6 +83,16 @@ static raw_spinlock_t *kretprobe_table_lock_ptr(unsigned long hash)
>  	return &(kretprobe_table_locks[hash].lock);
>  }
>  
> +/* List of symbols that can be overriden for error injection. */
> +static LIST_HEAD(kprobe_error_injection_list);
> +static DEFINE_MUTEX(kprobe_ei_mutex);
> +struct kprobe_ei_entry {
> +	struct list_head list;
> +	unsigned long start_addr;
> +	unsigned long end_addr;
> +	void *priv;
> +};
> +
>  /* Blacklist -- list of struct kprobe_blacklist_entry */
>  static LIST_HEAD(kprobe_blacklist);
>  
> @@ -1394,6 +1404,17 @@ bool within_kprobe_blacklist(unsigned long addr)
>  	return false;
>  }
>  
> +bool within_kprobe_error_injection_list(unsigned long addr)
> +{
> +	struct kprobe_ei_entry *ent;
> +
> +	list_for_each_entry(ent, &kprobe_error_injection_list, list) {
> +		if (addr >= ent->start_addr && addr < ent->end_addr)
> +			return true;
> +	}
> +	return false;
> +}
> +
>  /*
>   * If we have a symbol_name argument, look it up and add the offset field
>   * to it. This way, we can specify a relative address to a symbol.
> @@ -2168,6 +2189,86 @@ static int __init populate_kprobe_blacklist(unsigned long *start,
>  	return 0;
>  }
>  
> +#ifdef CONFIG_BPF_KPROBE_OVERRIDE
> +/* Markers of the _kprobe_error_inject_list section */
> +extern unsigned long __start_kprobe_error_inject_list[];
> +extern unsigned long __stop_kprobe_error_inject_list[];
> +
> +/*
> + * Lookup and populate the kprobe_error_injection_list.
> + *
> + * For safety reasons we only allow certain functions to be overriden with
> + * bpf_error_injection, so we need to populate the list of the symbols that have
> + * been marked as safe for overriding.
> + */
> +static void populate_kprobe_error_injection_list(unsigned long *start,
> +						 unsigned long *end,
> +						 void *priv)
> +{
> +	unsigned long *iter;
> +	struct kprobe_ei_entry *ent;
> +	unsigned long entry, offset = 0, size = 0;
> +
> +	mutex_lock(&kprobe_ei_mutex);
> +	for (iter = start; iter < end; iter++) {
> +		entry = arch_deref_entry_point((void *)*iter);
> +
> +		if (!kernel_text_address(entry) ||
> +		    !kallsyms_lookup_size_offset(entry, &size, &offset)) {
> +			pr_err("Failed to find error inject entry at %p\n",
> +				(void *)entry);
> +			continue;
> +		}
> +
> +		ent = kmalloc(sizeof(*ent), GFP_KERNEL);
> +		if (!ent)
> +			break;
> +		ent->start_addr = entry;
> +		ent->end_addr = entry + size;
> +		ent->priv = priv;
> +		INIT_LIST_HEAD(&ent->list);
> +		list_add_tail(&ent->list, &kprobe_error_injection_list);
> +	}
> +	mutex_unlock(&kprobe_ei_mutex);
> +}
> +
> +static void __init populate_kernel_kprobe_ei_list(void)
> +{
> +	populate_kprobe_error_injection_list(__start_kprobe_error_inject_list,
> +					     __stop_kprobe_error_inject_list,
> +					     NULL);
> +}
> +
> +static void module_load_kprobe_ei_list(struct module *mod)
> +{
> +	if (!mod->num_kprobe_ei_funcs)
> +		return;
> +	populate_kprobe_error_injection_list(mod->kprobe_ei_funcs,
> +					     mod->kprobe_ei_funcs +
> +					     mod->num_kprobe_ei_funcs, mod);
> +}
> +
> +static void module_unload_kprobe_ei_list(struct module *mod)
> +{
> +	struct kprobe_ei_entry *ent, *n;
> +	if (!mod->num_kprobe_ei_funcs)
> +		return;
> +
> +	mutex_lock(&kprobe_ei_mutex);
> +	list_for_each_entry_safe(ent, n, &kprobe_error_injection_list, list) {
> +		if (ent->priv == mod) {
> +			list_del_init(&ent->list);
> +			kfree(ent);
> +		}
> +	}
> +	mutex_unlock(&kprobe_ei_mutex);
> +}
> +#else
> +static inline void __init populate_kernel_kprobe_ei_list(void) {}
> +static inline void module_load_kprobe_ei_list(struct module *m) {}
> +static inline void module_unload_kprobe_ei_list(struct module *m) {}
> +#endif
> +
>  /* Module notifier call back, checking kprobes on the module */
>  static int kprobes_module_callback(struct notifier_block *nb,
>  				   unsigned long val, void *data)
> @@ -2178,6 +2279,11 @@ static int kprobes_module_callback(struct notifier_block *nb,
>  	unsigned int i;
>  	int checkcore = (val == MODULE_STATE_GOING);
>  
> +	if (val == MODULE_STATE_COMING)
> +		module_load_kprobe_ei_list(mod);
> +	else if (val == MODULE_STATE_GOING)
> +		module_unload_kprobe_ei_list(mod);
> +
>  	if (val != MODULE_STATE_GOING && val != MODULE_STATE_LIVE)
>  		return NOTIFY_DONE;
>  
> @@ -2240,6 +2346,8 @@ static int __init init_kprobes(void)
>  		pr_err("Please take care of using kprobes.\n");
>  	}
>  
> +	populate_kernel_kprobe_ei_list();
> +
>  	if (kretprobe_blacklist_size) {
>  		/* lookup the function address from its name */
>  		for (i = 0; kretprobe_blacklist[i].name != NULL; i++) {
> @@ -2407,6 +2515,56 @@ static const struct file_operations debugfs_kprobe_blacklist_ops = {
>  	.release        = seq_release,
>  };
>  
> +/*
> + * kprobes/error_injection_list -- shows which functions can be overriden for
> + * error injection.
> + * */
> +static void *kprobe_ei_seq_start(struct seq_file *m, loff_t *pos)
> +{
> +	mutex_lock(&kprobe_ei_mutex);
> +	return seq_list_start(&kprobe_error_injection_list, *pos);
> +}
> +
> +static void kprobe_ei_seq_stop(struct seq_file *m, void *v)
> +{
> +	mutex_unlock(&kprobe_ei_mutex);
> +}
> +
> +static void *kprobe_ei_seq_next(struct seq_file *m, void *v, loff_t *pos)
> +{
> +	return seq_list_next(v, &kprobe_error_injection_list, pos);
> +}
> +
> +static int kprobe_ei_seq_show(struct seq_file *m, void *v)
> +{
> +	char buffer[KSYM_SYMBOL_LEN];
> +	struct kprobe_ei_entry *ent =
> +		list_entry(v, struct kprobe_ei_entry, list);
> +
> +	sprint_symbol(buffer, ent->start_addr);
> +	seq_printf(m, "%s\n", buffer);
> +	return 0;
> +}
> +
> +static const struct seq_operations kprobe_ei_seq_ops = {
> +	.start = kprobe_ei_seq_start,
> +	.next  = kprobe_ei_seq_next,
> +	.stop  = kprobe_ei_seq_stop,
> +	.show  = kprobe_ei_seq_show,
> +};
> +
> +static int kprobe_ei_open(struct inode *inode, struct file *filp)
> +{
> +	return seq_open(filp, &kprobe_ei_seq_ops);
> +}
> +
> +static const struct file_operations debugfs_kprobe_ei_ops = {
> +	.open           = kprobe_ei_open,
> +	.read           = seq_read,
> +	.llseek         = seq_lseek,
> +	.release        = seq_release,
> +};
> +
>  static void arm_all_kprobes(void)
>  {
>  	struct hlist_head *head;
> @@ -2548,6 +2706,11 @@ static int __init debugfs_kprobe_init(void)
>  	if (!file)
>  		goto error;
>  
> +	file = debugfs_create_file("error_injection_list", 0444, dir, NULL,
> +				  &debugfs_kprobe_ei_ops);
> +	if (!file)
> +		goto error;
> +
>  	return 0;
>  
>  error:
> diff --git a/kernel/module.c b/kernel/module.c
> index dea01ac9cb74..bd695bfdc5c4 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3118,7 +3118,11 @@ static int find_module_sections(struct module *mod, struct load_info *info)
>  					     sizeof(*mod->ftrace_callsites),
>  					     &mod->num_ftrace_callsites);
>  #endif
> -
> +#ifdef CONFIG_BPF_KPROBE_OVERRIDE
> +	mod->kprobe_ei_funcs = section_objs(info, "_kprobe_error_inject_list",
> +					    sizeof(*mod->kprobe_ei_funcs),
> +					    &mod->num_kprobe_ei_funcs);
> +#endif
>  	mod->extable = section_objs(info, "__ex_table",
>  				    sizeof(*mod->extable), &mod->num_exentries);
>  
> -- 
> 2.7.5
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>

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