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

From: Miklos Szeredi <miklos@szeredi.hu>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v3 1/4] ovl: use insert_inode_locked4() to hash a newly created inode
Date: Thu, 17 May 2018 10:10:58 +0200	[thread overview]
Message-ID: <CAJfpegtC=FHdHAr5WKoMpX+aBTsMQB9Ffv0NfFouuSA3M08sqQ@mail.gmail.com> (raw)
In-Reply-To: <CAOQ4uxj5vtH2MDkhP5UP4Mc3CMgMJoYjULZcyv1jZmSJgkDpnw@mail.gmail.com>

On Thu, May 17, 2018 at 8:03 AM, Amir Goldstein <amir73il@gmail.com> wrote:
> On Tue, May 15, 2018 at 4:37 PM, Amir Goldstein <amir73il@gmail.com> wrote:
>> On Tue, May 15, 2018 at 4:23 PM, Vivek Goyal <vgoyal@redhat.com> 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.
>>
>>> I am wondering why can't we call ovl_get_inode() in object creation
>>> path. That should take care of race between creation path and file
>>> handle decode and only one of the paths will get to instantiate and
>>> initialize ovl_inode and other path will wait.
>>>
>>
>> I don't even want to think if what you wrote makes sense.
>> Remember that the use case we are talking about is quite imaginary.
>> Ensuring internal structures consistency in our code and returning
>> error to user is the right thing to do for imaginary use cases IMO.
>>
>
> Having being forced to think about it ;-), I think using ovl_get_inode()
> in create code does make a weird sort of sense.

Going through the same code-path very much makes sense.

> The reason it is weird is because we will always be throwing away
> the new inode that we allocated in ovl_create_object().
> AFAICS, if only reason we need to allocate new inode in
> ovl_create_object() is to calculate i_mode with inode_init_owner()
> and that calculation can be factored out to not need an inode.

Not the only reason: we don't want inode allocation to fail after
successful creation.  Solution: add a preallocated inode argument to
ovl_get_inode() and deal with allocation failure there.

> The reason it does makes sense to use ovl_get_inode(), is because
> the alternative that Miklos suggested is to insert the new inode on
> the very very likely use case and throw away the new inode in the
> very very unlikely use case of collision. That alternative creates a
> somewhat complicated code path that is rarely to never executed -
> not the best practice.

Agreed completely.

Thanks,
Miklos