KASAN: use-after-free Read in task_is_descendant

[Kees Cook]

On Mon, Oct 22, 2018 at 10:54 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> On 10/21, Tetsuo Handa wrote:
>>
>> On 2018/10/21 16:10, syzbot wrote:
>> > BUG: KASAN: use-after-free in __read_once_size include/linux/compiler.h:188 [inline]
>> > BUG: KASAN: use-after-free in task_is_descendant.part.2+0x610/0x670 security/yama/yama_lsm.c:295
>> > Read of size 8 at addr ffff8801c4666b20 by task syz-executor3/12722
>> >
>> > CPU: 1 PID: 12722 Comm: syz-executor3 Not tainted 4.19.0-rc8+ #70
>> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> > Call Trace:
>> >  __dump_stack lib/dump_stack.c:77 [inline]
>> >  dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>> >  print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>> >  kasan_report_error mm/kasan/report.c:354 [inline]
>> >  kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>> >  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>> >  __read_once_size include/linux/compiler.h:188 [inline]
>> >  task_is_descendant.part.2+0x610/0x670 security/yama/yama_lsm.c:295
>>
>> Do we need to hold
>>
>>   write_lock_irq(&tasklist_lock);
>>
>> rather than
>>
>>   rcu_read_lock();
>>
>> when accessing
>>
>>   "struct task_struct"->real_parent
>
> Well, if "task" is stable (can't exit), then I think
>
>         rcu_dereference(task->real_parent)
>
> is fine, we know that ->real_parent did not pass exit_notif() yet.
>
> However, task_is_descendant() looks unnecessarily complicated, it could be
>
>         static int task_is_descendant(struct task_struct *parent,
>                                       struct task_struct *child)
>         {
>                 int rc = 0;
>                 struct task_struct *walker;
>
>                 if (!parent || !child)
>                         return 0;
>
>                 rcu_read_lock();
>                 for (walker = child; walker->pid; walker = rcu_dereference(walker->real_parent))
>                         if (same_thread_group(parent, walker)) {
>                                 rc = 1;
>                                 break;
>                         }
>                 rcu_read_unlock();
>
>                 return rc;
>         }
>
> And again, I do not know how/if yama ensures that child is rcu-protected, perhaps
> task_is_descendant() needs to check pid_alive(child) right after rcu_read_lock() ?

task_is_descendant() is called under rcu_read_lock() in both
ptracer_exception_found() and yama_ptrace_access_check() so I don't
understand how any of the tasks could get freed? This is walking
group_leader and real_parent -- are these not stable under rcu_lock()?

-- 
Kees Cook