The entire scheme for managing net namespaces seems unsafe. We depend
on synchronization via pn->hash_lock, but have no guarantee of the
existence of the "net" object -- hence no way to ensure the existence
of the lock itself. This should be relatively easy to fix though as
we should be able to get/put the net namespace as we add remove
objects to/from the pppoe hash.
Once you solve this existence issue, the flush_lock can be eliminated
altogether since all of the relevant code paths already depend on a
write_lock_bh(&pn->hash_lock), and that's the lock that should be use
to protect the pppoe_dev field.
Another patch to follow later...
--
Michal Ostrowski
mostrows@xxxxxxxxx
On Mon, Oct 19, 2009 at 7:36 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> Michal Ostrowski a écrit :
>> Here's my theory on this after an inital look...
>>
>> Looking at the oops report and disassembly of the actual module binary
>> that caused the oops, one can deduce that:
>>
>> Execution was in pppoe_flush_dev(). %ebx contained the pointer "struct
>> pppox_sock *po", which is what we faulted on, excuting "cmp %eax, 0x190(%ebx)".
>> %ebx value was 0xffffffff (hence we got "NULL pointer dereference at 0x18f").
>>
>> At this point "i" (stored in %esi) is 15 (valid), meaning that we got a value
>> of 0xffffffff in pn->hash_table[i].
>>
>>>From this I'd hypothesize that the combination of dev_put() and release_sock()
>> may have allowed us to free "pn". At the bottom of the loop we alreayd
>> recognize that since locks are dropped we're responsible for handling
>> invalidation of objects, and perhaps that should be extended to "pn" as well.
>> --
>> Michal Ostrowski
>> mostrows@xxxxxxxxx
>>
>>
>
> Looking at this stuff, I do believe flush_lock protection is not
> properly done.
>
> At the end of pppoe_connect() for example we can find :
>
> err_put:
> if (po->pppoe_dev) {
> dev_put(po->pppoe_dev);
> po->pppoe_dev = NULL;
> }
>
> This is done without any protection, and can therefore clash with
> pppoe_flush_dev() :
>
> spin_lock(&flush_lock);
> po->pppoe_dev = NULL; /* ppoe_dev can already be NULL before this point */
> spin_unlock(&flush_lock);
>
> dev_put(dev); /* oops */
>
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