Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot_dirty()

[Baokun Li <libaokun1@huawei.com>]

On 2023/6/27 17:28, Jan Kara wrote:
> On Tue 27-06-23 17:08:27, Baokun Li wrote:
>> Hello!
>>
>> On 2023/6/27 16:34, Jan Kara wrote:
>>> Hello!
>>>
>>> On Mon 26-06-23 21:55:49, Baokun Li wrote:
>>>> On 2023/6/26 21:09, Jan Kara wrote:
>>>>> On Sun 25-06-23 15:56:10, Baokun Li wrote:
>>>>>>>> I think we can simply focus on the race between the DQ_ACTIVE_B flag and
>>>>>>>> the DQ_MOD_B flag, which is the core problem, because the same quota
>>>>>>>> should not have both flags. These two flags are protected by dq_list_lock
>>>>>>>> and dquot->dq_lock respectively, so it makes sense to add a
>>>>>>>> wait_on_dquot() to ensure the accuracy of DQ_ACTIVE_B.
>>>>>>> But the fundamental problem is not only the race with DQ_MOD_B setting. The
>>>>>>> dquot structure can be completely freed by the time
>>>>>>> dquot_claim_space_nodirty() calls dquot_mark_dquot_dirty() on it. That's
>>>>>>> why I think making __dquot_transfer() obey dquot_srcu rules is the right
>>>>>>> solution.
>>>>>> Yes, now I also think that making __dquot_transfer() obey dquot_srcu
>>>>>> rules is a better solution. But with inode->i_lock protection, why would
>>>>>> the dquot structure be completely freed?
>>>>> Well, when dquot_claim_space_nodirty() calls mark_all_dquot_dirty() it does
>>>>> not hold any locks (only dquot_srcu). So nothing prevents dquot_transfer()
>>>>> to go, swap dquot structure pointers and drop dquot references and after
>>>>> that mark_all_dquot_dirty() can use a stale pointer to call
>>>>> mark_dquot_dirty() on already freed memory.
>>>>>
>>>> No, this doesn't look like it's going to happen.  The
>>>> mark_all_dquot_dirty() uses a pointer array pointer, the dquot in the
>>>> array is dynamically changing, so after swap dquot structure pointers,
>>>> mark_all_dquot_dirty() uses the new pointer, and the stale pointer is
>>>> always destroyed after swap, so there is no case of using the stale
>>>> pointer here.
>>> There is a case - CPU0 can prefetch the values from dquots[] array into its
>>> local cache, then CPU1 can update the dquots[] array (these writes can
>>> happily stay in CPU1 store cache invisible to other CPUs) and free the
>>> dquots via dqput(). Then CPU0 can pass the prefetched dquot pointers to
>>> mark_dquot_dirty(). There are no locks or memory barries preventing CPUs
>>> from ordering instructions and memory operations like this in the code...
>>> You can read Documentation/memory-barriers.txt about all the perils current
>>> CPU architecture brings wrt coordination of memory accesses among CPUs ;)
>>>
>>> 								Honza
>> Got it!
>>
>> Sorry for misunderstanding you (I thought "completely freed" meant
>> dquot_destroy(), but you should have meant dquot_release()).
> Well, the dquot can even get to dquot_destroy(). There's nothing really
> preventing CPU2 going into memory reclaim and free the dquot in
> dqcache_shrink_scan() still before CPU0 even calls mark_dquot_dirty() on
> it. Sure such timing on real hardware is very unlikely but in a VM where a
> virtual CPU can get starved for a significant amount of time this could
> happen.
>
> 								Honza
Yes, invalidate_dquots() calling do_destroy_dquot() does not have this 
problem
because it calls synchronize_srcu(&dquot_srcu) in drop_dquot_ref() before.

However, calling do_destroy_dquot() from dqcache_shrink_scan() is not
protected, and calling dqcache_shrink_scan() after P3 execution will trigger
the UAF by calling do_destroy_dquot() twice, as shown in function graph 1
in the patch description; If dqcache_shrink_scan() is called after dquot is
added to free_dquots and before P3 is executed, the UAF may be
triggered in dquot_mark_dquot_dirty().

Thank you for your patient explanation!
The new version of the solution is almost complete, and is doing some stress
testing, which I will send out once it passes.
-- 
With Best Regards,
Baokun Li
.