Re: [PATCH v3 1/4] ovl: use insert_inode_locked4() to hash a newly created inode

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On Wed, May 16, 2018 at 11:51 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Wed, May 16, 2018 at 11:34 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> On Tue, May 15, 2018 at 3:37 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>> On Tue, May 15, 2018 at 4:23 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>>>> On Tue, May 15, 2018 at 01:26:09PM +0300, Amir Goldstein wrote:
>>>>> Currently, there is a small window where ovl_obtain_alias() can
>>>>> race with ovl_instantiate() and create two different overlay inodes
>>>>> with the same underlying real non-dir non-hardlink inode.
>>>>>
>>>>> The race requires an adversary to guess the file handle of the
>>>>> yet to be created upper inode and decode the guessed file handle
>>>>> after ovl_creat_real(), but before ovl_instantiate().
>>>>>
>>>>> This patch fixes the race, by using insert_inode_locked4() to add
>>>>> a newly created inode to icache.
>>>>>
>>>>> If the newly created inode apears to already exist in icache (hashed
>>>>> by the same real upper inode), we export this error to user instead
>>>>> of silently not hashing the new inode.
>>>>
>>>> So we might return an error to user saying operation failed, but still
>>>> create file on upper. Does that sound little odd?
>>>>
>>>
>>> Yes, but I don't see a better solution.
>>
>> Might be better to kick the other, offending inode out, instead of
>> returning an error.  It would also simplify the error handling.
>>
>> We can do that by creating an ovl_inode_test_kick() variant that
>> unhashes the inode on match.  Also needs insert_inode_locked4() to use
>> hlist_for_each_entry_safe() instead of hlist_for_each_entry().
>>
>
> Do you really think that this corner use case calls for such actions,
> as creating flavors of inode cache helpers?

Yes, if it simplifies error handling.

> Remember that the so called "offending" inode, is not offending in
> a way that is wrong or incomplete in any way.

Right, so what about just using that inode instead of erroring out?

Thanks,
Miklos




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