Re: For review: seccomp_user_notif(2) manual page
From: Michael Kerrisk (man-pages)
Date: Sun Oct 25 2020 - 12:33:56 EST
Hi Jann,
On 10/1/20 4:14 AM, Jann Horn wrote:
> On Thu, Oct 1, 2020 at 3:52 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>> On Thu, Oct 1, 2020 at 1:25 AM Tycho Andersen <tycho@tycho.pizza> wrote:
>>> On Thu, Oct 01, 2020 at 01:11:33AM +0200, Jann Horn wrote:
>>>> On Thu, Oct 1, 2020 at 1:03 AM Tycho Andersen <tycho@tycho.pizza> wrote:
>>>>> On Wed, Sep 30, 2020 at 10:34:51PM +0200, Michael Kerrisk (man-pages) wrote:
>>>>>> On 9/30/20 5:03 PM, Tycho Andersen wrote:
>>>>>>> On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
>>>>>>>> ┌─────────────────────────────────────────────────────┐
>>>>>>>> │FIXME │
>>>>>>>> ├─────────────────────────────────────────────────────┤
>>>>>>>> │From my experiments, it appears that if a SEC‐ │
>>>>>>>> │COMP_IOCTL_NOTIF_RECV is done after the target │
>>>>>>>> │process terminates, then the ioctl() simply blocks │
>>>>>>>> │(rather than returning an error to indicate that the │
>>>>>>>> │target process no longer exists). │
>>>>>>>
>>>>>>> Yeah, I think Christian wanted to fix this at some point,
>>>>>>
>>>>>> Do you have a pointer that discussion? I could not find it with a
>>>>>> quick search.
>>>>>>
>>>>>>> but it's a
>>>>>>> bit sticky to do.
>>>>>>
>>>>>> Can you say a few words about the nature of the problem?
>>>>>
>>>>> I remembered wrong, it's actually in the tree: 99cdb8b9a573 ("seccomp:
>>>>> notify about unused filter"). So maybe there's a bug here?
>>>>
>>>> That thing only notifies on ->poll, it doesn't unblock ioctls; and
>>>> Michael's sample code uses SECCOMP_IOCTL_NOTIF_RECV to wait. So that
>>>> commit doesn't have any effect on this kind of usage.
>>>
>>> Yes, thanks. And the ones stuck in RECV are waiting on a semaphore so
>>> we don't have a count of all of them, unfortunately.
>>>
>>> We could maybe look inside the wait_list, but that will probably make
>>> people angry :)
>>
>> The easiest way would probably be to open-code the semaphore-ish part,
>> and let the semaphore and poll share the waitqueue. The current code
>> kind of mirrors the semaphore's waitqueue in the wqh - open-coding the
>> entire semaphore would IMO be cleaner than that. And it's not like
>> semaphore semantics are even a good fit for this code anyway.
>>
>> Let's see... if we didn't have the existing UAPI to worry about, I'd
>> do it as follows (*completely* untested). That way, the ioctl would
>> block exactly until either there actually is a request to deliver or
>> there are no more users of the filter. The problem is that if we just
>> apply this patch, existing users of SECCOMP_IOCTL_NOTIF_RECV that use
>> an event loop and don't set O_NONBLOCK will be screwed. So we'd
>> probably also have to add some stupid counter in place of the
>> semaphore's counter that we can use to preserve the old behavior of
>> returning -ENOENT once for each cancelled request. :(
>>
>> I guess this is a nice point in favor of Michael's usual complaint
>> that if there are no man pages for a feature by the time the feature
>> lands upstream, there's a higher chance that the UAPI will suck
>> forever...
>
> And I guess this would be the UAPI-compatible version - not actually
> as terrible as I thought it might be. Do y'all want this? If so, feel
> free to either turn this into a proper patch with Co-developed-by, or
> tell me that I should do it and I'll try to get around to turning it
> into something proper.
Thanks for taking a shot at this.
I tried applying the patch below to vanilla 5.9.0.
(There's one typo: s/ENOTCON/ENOTCONN).
It seems not to work though; when I send a signal to my test
target process that is sleeping waiting for the notification
response, the process enters the uninterruptible D state.
Any thoughts?
Thanks,
Michael
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 676d4af62103..d08c453fcc2c 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -138,7 +138,7 @@ struct seccomp_kaddfd {
> * @notifications: A list of struct seccomp_knotif elements.
> */
> struct notification {
> - struct semaphore request;
> + bool canceled_reqs;
> u64 next_id;
> struct list_head notifications;
> };
> @@ -859,7 +859,6 @@ static int seccomp_do_user_notification(int this_syscall,
> list_add(&n.list, &match->notif->notifications);
> INIT_LIST_HEAD(&n.addfd);
>
> - up(&match->notif->request);
> wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
> mutex_unlock(&match->notify_lock);
>
> @@ -901,8 +900,20 @@ static int seccomp_do_user_notification(int this_syscall,
> * *reattach* to a notifier right now. If one is added, we'll need to
> * keep track of the notif itself and make sure they match here.
> */
> - if (match->notif)
> + if (match->notif) {
> list_del(&n.list);
> +
> + /*
> + * We are stuck with a UAPI that requires that after a spurious
> + * wakeup, SECCOMP_IOCTL_NOTIF_RECV must return immediately.
> + * This is the tracking for that, keeping track of whether we
> + * canceled a request after waking waiters, but before userspace
> + * picked up the notification.
> + */
> + if (n.state == SECCOMP_NOTIFY_INIT)
> + match->notif->canceled_reqs = true;
> + }
> +
> out:
> mutex_unlock(&match->notify_lock);
>
> @@ -1178,6 +1189,7 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> void __user *buf)
> {
> struct seccomp_knotif *knotif = NULL, *cur;
> + DECLARE_WAITQUEUE(wait, current);
> struct seccomp_notif unotif;
> ssize_t ret;
>
> @@ -1190,11 +1202,9 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
>
> memset(&unotif, 0, sizeof(unotif));
>
> - ret = down_interruptible(&filter->notif->request);
> - if (ret < 0)
> - return ret;
> -
> mutex_lock(&filter->notify_lock);
> +
> +retry:
> list_for_each_entry(cur, &filter->notif->notifications, list) {
> if (cur->state == SECCOMP_NOTIFY_INIT) {
> knotif = cur;
> @@ -1202,14 +1212,32 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> }
> }
>
> - /*
> - * If we didn't find a notification, it could be that the task was
> - * interrupted by a fatal signal between the time we were woken and
> - * when we were able to acquire the rw lock.
> - */
> if (!knotif) {
> - ret = -ENOENT;
> - goto out;
> + /* This has to happen before checking &filter->users. */
> + prepare_to_wait(&filter->wqh, &wait, TASK_INTERRUPTIBLE);
> +
> + /*
> + * If all users of the filter are gone, throw an error instead
> + * of pointlessly continuing to block.
> + */
> + if (refcount_read(&filter->users) == 0) {
> + ret = -ENOTCON;
> + goto out;
> + }
> + if (filter->notif->canceled_reqs) {
> + ret = -ENOENT;
> + goto out;
> + } else {
> + /* No notifications pending - wait for one,
> then retry. */
> + mutex_unlock(&filter->notify_lock);
> + schedule();
> + mutex_lock(&filter->notify_lock);
> + if (signal_pending(current)) {
> + ret = -EINTR;
> + goto out;
> + }
> + goto retry;
> + }
> }
>
> unotif.id = knotif->id;
> @@ -1220,6 +1248,8 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> wake_up_poll(&filter->wqh, EPOLLOUT | EPOLLWRNORM);
> ret = 0;
> out:
> + filter->notif->canceled_reqs = false;
> + finish_wait(&filter->wqh, &wait);
> mutex_unlock(&filter->notify_lock);
>
> if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
> @@ -1233,10 +1263,8 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> */
> mutex_lock(&filter->notify_lock);
> knotif = find_notification(filter, unotif.id);
> - if (knotif) {
> + if (knotif)
> knotif->state = SECCOMP_NOTIFY_INIT;
> - up(&filter->notif->request);
> - }
> mutex_unlock(&filter->notify_lock);
> }
>
> @@ -1485,7 +1513,6 @@ static struct file *init_listener(struct
> seccomp_filter *filter)
> if (!filter->notif)
> goto out;
>
> - sema_init(&filter->notif->request, 0);
> filter->notif->next_id = get_random_u64();
> INIT_LIST_HEAD(&filter->notif->notifications);
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/