yama: unsafe usage of ptrace_relation->tracer

[Oleg Nesterov <oleg@redhat.com>]

let me change the subject to avoid the confusion with the already confusing
disccussion about task_is_descendant().

On 10/29, Oleg Nesterov wrote:
>
> I still think we need a single pid_alive() check and I even sent the patch.
> Attached again at the end.
>
> To clarify, let me repeat that ptracer_exception_found() may need some fixes
> too, right now I am only talking about task_is_descendant().

so yes, the ptracer_relations code looks very broken to me, but perhaps I
misread this code, please correct me.

RCU can only protect the ptracer_relations list itself, you can do nothing
with (say) relation->tracer. relation->tracer can be already freed when
ptracer_exception_found() checks relation->tracee == tracee.

Not only pid_alive(parent) can not help in this case, pid_alive(parent) is
equally unsafe because, again, this memory can be freed.

security_task_free(tsk) is called right before free_task(tsk), there is no
a gp pass in between, and of course we can't rely on the ->invalid check.

_At first glance_ we can fix this if we simply turn both ->tracer/tracee
pointers into "signal_struct *", then we can turn all same_thread_group()'s
into walker->signal == parent which doesn't need to dereference the possibly-
freed parent. This also allows to remove all thread_group_leader() checks.
We need to ensure that false-positive is not possible (if, say, ->tracer
was already re-allocated and points to another task->signal), but this
doesn't look difficult.

Oleg.