Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

From: Kees Cook
Date: Tue Jun 24 2014 - 14:05:31 EST


On Tue, Jun 24, 2014 at 10:27 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/23, Kees Cook wrote:
>>
>> +static pid_t seccomp_can_sync_threads(void)
>> +{
>> + struct task_struct *thread, *caller;
>> +
>> + BUG_ON(write_can_lock(&tasklist_lock));
>> + BUG_ON(!spin_is_locked(&current->sighand->siglock));
>> +
>> + if (current->seccomp.mode != SECCOMP_MODE_FILTER)
>> + return -EACCES;
>> +
>> + /* Validate all threads being eligible for synchronization. */
>> + thread = caller = current;
>> + for_each_thread(caller, thread) {
>> + pid_t failed;
>> +
>> + if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
>> + (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
>> + is_ancestor(thread->seccomp.filter,
>> + caller->seccomp.filter)))
>> + continue;
>> +
>> + /* Return the first thread that cannot be synchronized. */
>> + failed = task_pid_vnr(thread);
>> + /* If the pid cannot be resolved, then return -ESRCH */
>> + if (failed == 0)
>> + failed = -ESRCH;
>
> forgot to mention, task_pid_vnr() can't fail. sighand->siglock is held,
> for_each_thread() can't see a thread which has passed unhash_process().

Certainly good to know, but I'd be much more comfortable leaving this
check as-is. Having "failed" return with "0" would be very very bad
(userspace would think the filter had been successfully applied, etc).
I'd rather stay highly defensive here.

-Kees

--
Kees Cook
Chrome OS Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/