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SubjectRe: [PATCH 3/6] tracing: Change event_filter_read/write to verify i_private != NULL
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On Tue, 2013-07-23 at 22:59 +0200, Oleg Nesterov wrote:
> event_filter_read/write() are racy, ftrace_event_call can be already
> freed by trace_remove_event_call() callers.
>
> 1. Shift mutex_lock(event_mutex) from print/apply_event_filter to
> the callers. Rename print/apply just in case.

I wouldn't do the rename unless there is a print_/apply_ version added
later. Just add a comment that event_mutex must be held for those
functions.

>
> 2. Change the callers, event_filter_read() and event_filter_write()
> to read i_private under this mutex and abort if it is NULL.
>
> This fixes nothing, but now we can change debugfs_remove("filter")
> callers to nullify ->i_private and fix the the problem.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> kernel/trace/trace.h | 4 ++--
> kernel/trace/trace_events.c | 28 ++++++++++++++++++----------
> kernel/trace/trace_events_filter.c | 21 ++++++++-------------
> 3 files changed, 28 insertions(+), 25 deletions(-)
>
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index afaae41..8b7c72b 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -985,9 +985,9 @@ struct filter_pred {
>
> extern enum regex_type
> filter_parse_regex(char *buff, int len, char **search, int *not);
> -extern void print_event_filter(struct ftrace_event_call *call,
> +extern void __print_event_filter(struct ftrace_event_call *call,
> struct trace_seq *s);
> -extern int apply_event_filter(struct ftrace_event_call *call,
> +extern int __apply_event_filter(struct ftrace_event_call *call,
> char *filter_string);
> extern int apply_subsystem_event_filter(struct ftrace_subsystem_dir *dir,
> char *filter_string);
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 821768e..0f081c0 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -977,9 +977,9 @@ static ssize_t
> event_filter_read(struct file *filp, char __user *ubuf, size_t cnt,
> loff_t *ppos)
> {
> - struct ftrace_event_call *call = filp->private_data;
> + struct ftrace_event_call *call;
> struct trace_seq *s;
> - int r;
> + int r = -ENODEV;
>
> if (*ppos)
> return 0;
> @@ -987,14 +987,18 @@ event_filter_read(struct file *filp, char __user *ubuf, size_t cnt,
> s = kmalloc(sizeof(*s), GFP_KERNEL);
> if (!s)
> return -ENOMEM;
> -

Again, I prefer she space. Just my preference.

> trace_seq_init(s);
>
> - print_event_filter(call, s);
> - r = simple_read_from_buffer(ubuf, cnt, ppos, s->buffer, s->len);
> + mutex_lock(&event_mutex);
> + call = event_file_data(filp);
> + if (likely(call))

This isn't a fast path. Remove the "likely". It just makes the code
ugly.

> + __print_event_filter(call, s);
> + mutex_unlock(&event_mutex);
>
> - kfree(s);
> + if (call)

Especially since we have no likely here.

> + r = simple_read_from_buffer(ubuf, cnt, ppos, s->buffer, s->len);
>
> + kfree(s);
> return r;
> }
>
> @@ -1002,9 +1006,9 @@ static ssize_t
> event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
> loff_t *ppos)
> {
> - struct ftrace_event_call *call = filp->private_data;
> + struct ftrace_event_call *call;
> char *buf;
> - int err;
> + int err = -ENODEV;
>
> if (cnt >= PAGE_SIZE)
> return -EINVAL;
> @@ -1019,13 +1023,17 @@ event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
> }
> buf[cnt] = '\0';
>
> - err = apply_event_filter(call, buf);
> + mutex_lock(&event_mutex);
> + call = event_file_data(filp);
> + if (likely(call))

Again, remove the likely.

> + err = __apply_event_filter(call, buf);
> + mutex_unlock(&event_mutex);
> +
> free_page((unsigned long) buf);
> if (err < 0)
> return err;
>
> *ppos += cnt;
> -
> return cnt;
> }
>
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 0c7b75a..71f76ee 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -637,17 +637,15 @@ static void append_filter_err(struct filter_parse_state *ps,
> free_page((unsigned long) buf);
> }
>
> -void print_event_filter(struct ftrace_event_call *call, struct trace_seq *s)
> +/* caller must hold event_mutex */
> +void __print_event_filter(struct ftrace_event_call *call, struct trace_seq *s)
> {
> - struct event_filter *filter;
> + struct event_filter *filter = call->filter;
>
> - mutex_lock(&event_mutex);
> - filter = call->filter;
> if (filter && filter->filter_string)
> trace_seq_printf(s, "%s\n", filter->filter_string);
> else
> trace_seq_puts(s, "none\n");
> - mutex_unlock(&event_mutex);
> }
>
> void print_subsystem_event_filter(struct event_subsystem *system,
> @@ -1841,23 +1839,22 @@ static int create_system_filter(struct event_subsystem *system,
> return err;
> }
>
> -int apply_event_filter(struct ftrace_event_call *call, char *filter_string)
> +/* caller must hold event_mutex */
> +int __apply_event_filter(struct ftrace_event_call *call, char *filter_string)
> {
> struct event_filter *filter;
> int err = 0;

We can remove the 0 init here.

>
> - mutex_lock(&event_mutex);
> -
> if (!strcmp(strstrip(filter_string), "0")) {
> filter_disable(call);
> filter = call->filter;
> if (!filter)
> - goto out_unlock;
> + goto out;

Remove the "out" label, and just return 0 here.

> RCU_INIT_POINTER(call->filter, NULL);
> /* Make sure the filter is not being used */
> synchronize_sched();
> __free_filter(filter);
> - goto out_unlock;
> + goto out;

Here too.

> }
>
> err = create_filter(call, filter_string, true, &filter);
> @@ -1884,9 +1881,7 @@ int apply_event_filter(struct ftrace_event_call *call, char *filter_string)
> __free_filter(tmp);
> }
> }
> -out_unlock:
> - mutex_unlock(&event_mutex);
> -
> +out:
> return err;
> }
>

-- Steve




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