Re: [PATCH v4 9/9] ovl: use iget5_prealloc() to hash a newly created inode

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On Fri, May 18, 2018 at 5:03 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, May 18, 2018 at 11:29:37AM +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 race does not affect overlay directory inodes, because those
>> are decoded via ovl_lookup_real() and not with ovl_obtain_alias().
>>
>> This patch fixes the race, by using iget5_prealloc() to add a newly
>> created inode to cache.
>
> Mind explaining what the hell is wrong with insert_inode_locked4()?

That it doesn't return the old inode if found.

Btw, these functions are eerily similar, and it'd be nice to reduce
the number of them.

Thanks,
Miklos
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