Re: [Xenomai-core] Potential heap corruption on thread cleanup

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Hi Gilles,
>>>
>>> I'm pushing your findings to the list, also as my colleagues showed
>>> strong interest - this thing may explain rare corruptions for us as well.
>>>
>>> I thought a bit about that likely u_mode-related crash in your test case
>>> and have the following theory so far: If the xeno_current_mode storage
>>> is allocated on the application heap (!HAVE_THREAD, that's also what we
>>> are forced to use), it is automatically freed on thread termination in
>>> the context of the dying thread. If the thread is already migrated to
>>> secondary or if that happens while it is cleaned up (i.e. before calling
>>> for exit into the kernel), there is no problem, Xenomai will not touch
>>> the mode storage anymore. But if the thread happens to delete the
>>> storage "silently", without any migration, the final exit will trigger
>>> one further access. And that takes place against an invalid head area at
>>> this point.
>>>
>>> Does this make sense?
>> Yes, it is the issue we observed.
>>
>>> If that is true, all we need to do is to force a migration before
>>> releasing the mode storage. Could you check this?
>> No, that does not fly. Calling, for instance, __wrap_pthread_mutex_lock
>> in another TSD cleanup function is which could be called after the
>> current_mode TSD cleanup is allowed and could trigger a switch to
>> primary mode and a write to the u_mode.
>>
> 
> Good point. Mmh. Another, but ABI-breaking, way would be to add a
> syscall for deregistering the u_mode pointer...

That is the thing we did to verify that we had this bug. But this
syscall would be also called too soon, and suffers from the TSD cleanup
functions order again.

-- 
					    Gilles.