Re: [RFC PATCH] exec: Avoid recursive modprobe for binary format handlers

[Kees Cook]

On Wed, Aug 2, 2017 at 4:23 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> On Tue, Aug 01, 2017 at 07:28:20PM -0700, Kees Cook wrote:
>> On Tue, Aug 1, 2017 at 5:12 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
>> > On Fri, Jul 21, 2017 at 03:05:20PM +0100, Matt Redfearn wrote:
>> >> Commit 6d7964a722af ("kmod: throttle kmod thread limit") which was
>> >> merged in v4.13-rc1 broke this behaviour since the recursive modprobe is
>> >> no longer caught, it just ends up waiting indefinitely for the kmod_wq
>> >> wait queue. Hence the kernel appears to hang silently when starting
>> >> userspace.
>> >
>> > Indeed, the recursive issue were no longer expected to exist.
>>
>> Errr, yeah, recursive binfmt loads can still happen.
>>
>> > The *old* implementation would also prevent a set of binaries to daisy chain
>> > a set of 50 different binaries which require different binfmt loaders. The
>> > current implementation enables this and we'd just wait. There's a bound to
>> > the number of binfmd loaders though, so this would be bounded. If however
>> > a 2nd loader loaded the first binary we'd run into the same issue I think.
>> >
>> > If we can't think of a good way to resolve this we'll just have to revert
>> > 6d7964a722af for now.
>>
>> The weird but "normal" recursive case is usually a script calling a
>> script calling a misc format. Getting a chain of modprobes running,
>> though, seems unlikely. I *think* Matt's patch is okay, but I agree,
>> it'd be better for the request_module() to fail.
>
> In that case how about we just have each waiter only wait max X seconds,
> if the number of concurrent ongoing modprobe calls hasn't reduced by
> a single digit in X seconds we give up on request_module() for the
> module and clearly indicate what happened.
>
> Matt, can you test?
>
> Note I've used wait_event_killable_timeout() to only accept SIGKILL
> for now. I've seen issues wit SIGCHILD and at modprobe this could
> even be a bigger issue, so this would restrict the signals received
> *only* to SIGKILL.
>
> It would be good to come up with a simple test case for this in
> tools/testing/selftests/kmod/kmod.sh
>
>   Luis
>
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 5b74e36c0ca8..dc19880c02f5 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -757,6 +757,43 @@ extern int do_wait_intr_irq(wait_queue_head_t *, wait_queue_entry_t *);
>         __ret;                                                                  \
>  })
>
> +#define __wait_event_killable_timeout(wq_head, condition, timeout)             \
> +       ___wait_event(wq_head, ___wait_cond_timeout(condition),                 \
> +                     TASK_KILLABLE, 0, timeout,                                \
> +                     __ret = schedule_timeout(__ret))
> +
> +/**
> + * wait_event_killable_timeout - sleep until a condition gets true or a timeout elapses
> + * @wq_head: the waitqueue to wait on
> + * @condition: a C expression for the event to wait for
> + * @timeout: timeout, in jiffies
> + *
> + * The process is put to sleep (TASK_KILLABLE) until the
> + * @condition evaluates to true or a kill signal is received.
> + * The @condition is checked each time the waitqueue @wq_head is woken up.
> + *
> + * wake_up() has to be called after changing any variable that could
> + * change the result of the wait condition.
> + *
> + * Returns:
> + * 0 if the @condition evaluated to %false after the @timeout elapsed,
> + * 1 if the @condition evaluated to %true after the @timeout elapsed,
> + * the remaining jiffies (at least 1) if the @condition evaluated
> + * to %true before the @timeout elapsed, or -%ERESTARTSYS if it was
> + * interrupted by a kill signal.
> + *
> + * Only kill signals interrupt this process.
> + */
> +#define wait_event_killable_timeout(wq_head, condition, timeout)               \
> +({                                                                             \
> +       long __ret = timeout;                                                   \
> +       might_sleep();                                                          \
> +       if (!___wait_cond_timeout(condition))                                   \
> +               __ret = __wait_event_killable_timeout(wq_head,                  \
> +                                               condition, timeout);            \
> +       __ret;                                                                  \
> +})
> +
>
>  #define __wait_event_lock_irq(wq_head, condition, lock, cmd)                   \
>         (void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,     \
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 6d016c5d97c8..1b5f7bada8d2 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -71,6 +71,13 @@ static atomic_t kmod_concurrent_max = ATOMIC_INIT(MAX_KMOD_CONCURRENT);
>  static DECLARE_WAIT_QUEUE_HEAD(kmod_wq);
>
>  /*
> + * If modprobe can't be called after this time we assume its very likely
> + * your userspace has created a recursive dependency, and we'll have no
> + * option but to fail.
> + */
> +#define MAX_KMOD_TIMEOUT 5

Would this mean slow (swappy) systems could start failing modprobe
just due to access times?

-Kees

> +
> +/*
>         modprobe_path is set via /proc/sys.
>  */
>  char modprobe_path[KMOD_PATH_LEN] = "/sbin/modprobe";
> @@ -167,8 +174,18 @@ int __request_module(bool wait, const char *fmt, ...)
>                 pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
>                                     atomic_read(&kmod_concurrent_max),
>                                     MAX_KMOD_CONCURRENT, module_name);
> -               wait_event_interruptible(kmod_wq,
> -                                        atomic_dec_if_positive(&kmod_concurrent_max) >= 0);
> +               ret = wait_event_killable_timeout(kmod_wq,
> +                                                 atomic_dec_if_positive(&kmod_concurrent_max) >= 0,
> +                                                 MAX_KMOD_TIMEOUT * HZ);
> +               if (!ret) {
> +                       pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
> +                                           module_name, atomic_read(&kmod_concurrent_max), MAX_KMOD_TIMEOUT);
> +                       pr_warn_ratelimited("request_module: recursive modprobe call very likely!");
> +                       return -ETIME;
> +               } else if (ret == -ERESTARTSYS) {
> +                       pr_warn_ratelimited("request_module: sigkill sent for modprobe %s, giving up", module_name);
> +                       return ret;
> +               }
>         }
>
>         trace_module_request(module_name, wait, _RET_IP_);



-- 
Kees Cook
Pixel Security