Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe

Anders Roxell
2019-12-02T15:17:43Z

On Mon, 2 Dec 2019 at 08:32, Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Anders reported that the lockdep warns that suspicious
> RCU list usage in register_kprobe() (detected by
> CONFIG_PROVE_RCU_LIST.) This is because get_kprobe()
> access kprobe_table[] by hlist_for_each_entry_rcu()
> without rcu_read_lock.
>
> If we call get_kprobe() from the breakpoint handler context,
> it is run with preempt disabled, so this is not a problem.
> But in other cases, instead of rcu_read_lock(), we locks
> kprobe_mutex so that the kprobe_table[] is not updated.
> So, current code is safe, but still not good from the view
> point of RCU.
>
> Let's lock the rcu_read_lock() around get_kprobe() and
> ensure kprobe_mutex is locked at those points.
>
> Note that we can safely unlock rcu_read_lock() soon after
> accessing the list, because we are sure the found kprobe has
> never gone before unlocking kprobe_mutex. Unless locking
> kprobe_mutex, caller must hold rcu_read_lock() until it
> finished operations on that kprobe.
>
> Reported-by: Anders Roxell <anders.roxell@linaro.org>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you Masami for fixing this.

Tested-by: Anders Roxell <anders.roxell@linaro.org>

Cheers,
Anders

> ---
> kernel/kprobes.c | 18 ++++++++++++++----
> 1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 53534aa258a6..fd814ea7dbd8 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -319,6 +319,7 @@ static inline void reset_kprobe_instance(void)
> * - under the kprobe_mutex - during kprobe_[un]register()
> * OR
> * - with preemption disabled - from arch/xxx/kernel/kprobes.c
> + * In both cases, caller must disable preempt (or acquire rcu_read_lock)
> */
> struct kprobe *get_kprobe(void *addr)
> {
> @@ -435,6 +436,7 @@ static int kprobe_queued(struct kprobe *p)
> /*
> * Return an optimized kprobe whose optimizing code replaces
> * instructions including addr (exclude breakpoint).
> + * This must be called with locking kprobe_mutex.
> */
> static struct kprobe *get_optimized_kprobe(unsigned long addr)
> {
> @@ -442,9 +444,12 @@ static struct kprobe *get_optimized_kprobe(unsigned long addr)
> struct kprobe *p = NULL;
> struct optimized_kprobe *op;
>
> + lockdep_assert_held(&kprobe_mutex);
> + rcu_read_lock();
> /* Don't check i == 0, since that is a breakpoint case. */
> for (i = 1; !p && i < MAX_OPTIMIZED_LENGTH; i++)
> p = get_kprobe((void *)(addr - i));
> + rcu_read_unlock(); /* We are safe because kprobe_mutex is held */
>
> if (p && kprobe_optready(p)) {
> op = container_of(p, struct optimized_kprobe, kp);
> @@ -1478,18 +1483,21 @@ static struct kprobe *__get_valid_kprobe(struct kprobe *p)
> {
> struct kprobe *ap, *list_p;
>
> + lockdep_assert_held(&kprobe_mutex);
> + rcu_read_lock();
> ap = get_kprobe(p->addr);
> if (unlikely(!ap))
> - return NULL;
> + goto out;
>
> if (p != ap) {
> list_for_each_entry_rcu(list_p, &ap->list, list)
> if (list_p == p)
> /* kprobe p is a valid probe */
> - goto valid;
> - return NULL;
> + goto out;
> + ap = NULL;
> }
> -valid:
> +out:
> + rcu_read_unlock(); /* We are safe because kprobe_mutex is held */
> return ap;
> }
>
> @@ -1602,7 +1610,9 @@ int register_kprobe(struct kprobe *p)
>
> mutex_lock(&kprobe_mutex);
>
> + rcu_read_lock();
> old_p = get_kprobe(p->addr);
> + rcu_read_unlock(); /* We are safe because kprobe_mutex is held */
> if (old_p) {
> /* Since this may unoptimize old_p, locking text_mutex. */
> ret = register_aggr_kprobe(old_p, p);
>