Messages in this thread | | | Date | Tue, 27 Jun 2023 22:06:47 +0800 | Subject | Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot_dirty() | From | Baokun Li <> |
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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 .
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