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SubjectRe: [PATCH v2 1/2] seccomp: notify user trap about unused filter
On Thu, May 28, 2020 at 04:11:00PM -0700, Kees Cook wrote:
> On Thu, May 28, 2020 at 05:14:11PM +0200, Christian Brauner wrote:
> > * @usage: reference count to manage the object lifetime.
> > * get/put helpers should be used when accessing an instance
> > * outside of a lifetime-guarded section. In general, this
> > * is only needed for handling filters shared across tasks.
> > [...]
> > + * @live: Number of tasks that use this filter directly and number
> > + * of dependent filters that have a non-zero @live counter.
> > + * Altered during fork(), exit(), and filter installation
> > [...]
> > refcount_set(&sfilter->usage, 1);
> > + refcount_set(&sfilter->live, 1);
>
> I'd like these reference counters to have more descriptive names. "usage"
> by what? "live" from what perspective? At the least, I think we need
> to be explicit in the comment, and at best we should do that and rename
> them to be a bit more clear.

Well the correct way would probably be:
"usage" -> "refs"
"live" -> "users"

So we'd need a first patch to convert "usage" to "refs" and then
introduce "users".

>
> A filter's "usage" is incremented for each directly-attached task
> (task::seccomp_data.filter, via fork() or thread_sync), once for the
> dependent filter (filter::prev), and once for an open user_notif file
> (file::private_data). When it reaches zero, there are (should be) no more
> active memory references back to the struct filter and it can be freed.
>
> A filter's "live" is incremented for each directly-attached task
> (task::seccomp_data.filter, via fork() or thread_sync), and once for
> the dependent filter (filter::prev). When it reaches zero there is no
> way for new tasks to get associated with the filter, but there may still
> be user_notif file::private_data references pointing at the filter.

or - at least briefyl - ptrace or whatever, yes.

>
> But we're tracking "validity lifetime" (live) and "memory reference
> safety" (usage).
>
> signal_struct has "sigcnt" and "live". I find "sigcnt" to be an
> unhelpful name too. (And why isn't it refcount_t?)

I think I once looked that up and there was some sort of "not needed, no
gain" style rationale.

>
> So, perhaps leave "live", but rename "usage" -> "references".

usage -> refs
live -> users/active

>
> After looking at these other lifetime management examples in the kernel,
> I'm convinced that tracking these states separately is correct, but I
> remain uncomfortable about task management needing to explicitly make
> two calls to let go of the filter.
>
> I wonder if release_task() should also detach the filter from the task
> and do a put_seccomp_filter() instead of waiting for task_free(). This
> is supported by the other place where seccomp_filter_release() is
> called:
>
> > @@ -396,6 +400,7 @@ static inline void seccomp_sync_threads(unsigned long flags)
> > * allows a put before the assignment.)
> > */
> > put_seccomp_filter(thread);
> > + seccomp_filter_release(thread);
>
> This would also remove the only put_seccomp_filter() call outside of
> seccomp.c, since the free_task() call will be removed now in favor of
> the task_release() call.
>
> So, is it safe to detach the filter in release_task()? Has dethreading
> happened yet? i.e. can we race TSYNC? -- is there a possible
> inc-from-zero? (Actually, all our refcount_inc()s should be
> refcount_inc_not_zero() just for robustness.) I *think* we can do it
> before the release_thread() call (instead of after cgroup_release()).
>
> With that, then seccomp_filter_release() could assign the filter to NULL
> and do the clean up:
>
> void seccomp_filter_release(const struct task_struct *tsk)
> {
> struct seccomp_filter *orig = READ_ONCE(tsk->seccomp.filter);
>
> smp_store_release(&tsk->seccomp.filter, NULL);
> __seccomp_filter_release(orig);
> }
>
> All other refcounting is then internal to seccomp.c. Which brings me
> back to TSYNC, since we don't want to write NULL to task->seccomp.filter
> during TSYNC. TSYNC can use:
>
> void __seccomp_filter_release(struct seccomp_filter *filter)
> {
> while (filter && refcount_dec_and_test(&filter->live)) {
> if (waitqueue_active(&filter->wqh))
> wake_up_poll(&filter->wqh, EPOLLHUP);
> filter = filter->prev;
> }
> __put_seccomp_filter(filter);
> }
>
> Thoughts?
>
> --
> Kees Cook

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