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SubjectRe: [PATCH 2/2] sys, seccomp: add PR_SECCOMP_EXT and SECCOMP_EXT_ACT_TSYNC
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On Tue, Jan 14, 2014 at 1:21 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 01/13, Will Drewry wrote:
>>
>> +static pid_t seccomp_sync_threads(void)
>> +{
>> + struct task_struct *thread, *caller;
>> + pid_t failed = 0;
>> + thread = caller = current;
>> +
>> + read_lock(&tasklist_lock);
>> + if (thread_group_empty(caller))
>> + goto done;
>> + while_each_thread(caller, thread) {
>> + task_lock(thread);
>
> perhaps we take task_lock() to serialize with another caller of
> seccomp_sync_threads()...

Sorry for the patch being unclear! The task_lock is meant to protect
against the assignment of a seccomp.filter pointer which was meant to
protect against the target task calling seccomp_attach_filter while
another task is changing its filter pointer.

> If yes, then perhaps you can use ->siglock instead of tasklist_lock
> and do not use task_lock(). It would be even better to rely on rcu,
> but:

Exactly. The siglock seems like it makes more sense.

>> + get_seccomp_filter(caller);
>> + /*
>> + * Drop the task reference to the shared ancestor since
>> + * current's path will hold a reference. (This also
>> + * allows a put before the assignment.)
>> + */
>> + put_seccomp_filter(thread);
>> + thread->seccomp.filter = caller->seccomp.filter;
>
> As I said, I do not understand this patch yet, but this looks suspicious.
>
> Why we can't race with this thread doing clone(CLONE_THREAD) ? We do
> not the the new thread yet, but its ->seccomp can be already copied
> by copy_process(), no?

Yeah I missed that. That said, I think the worst of it would be that
the new thread
gets the old filter. It should still get_seccomp_filter() its own
reference to the filter.
I'm not clear if it's possible to ensure that there is no pathological
condition where
a thread races and is created without being synchronized. I'll see if
the siglock helps
here and walk the clone() code again to see what else I missed.

thanks!


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