Re: IORING_REGISTER_CREDS[_UPDATE]() and credfd_create()?

Jens Axboe
2020-01-28T19:42:36Z

On 1/28/20 11:04 AM, Jens Axboe wrote:
> On 1/28/20 10:19 AM, Jens Axboe wrote:
>> On 1/28/20 9:19 AM, Jens Axboe wrote:
>>> On 1/28/20 9:17 AM, Stefan Metzmacher wrote:
>>>> Am 28.01.20 um 17:10 schrieb Jens Axboe:
>>>>> On 1/28/20 3:18 AM, Stefan Metzmacher wrote:
>>>>>> Hi Jens,
>>>>>>
>>>>>> now that we have IORING_FEAT_CUR_PERSONALITY...
>>>>>>
>>>>>> How can we optimize the fileserver case now, in order to avoid the
>>>>>> overhead of always calling 5 syscalls before io_uring_enter()?:
>>>>>>
>>>>>> /* gain root again */
>>>>>> setresuid(-1,0,-1); setresgid(-1,0,-1)
>>>>>> /* impersonate the user with groups */
>>>>>> setgroups(num, grps); setresgid(-1,gid,-1); setresuid(-1,uid,-1);
>>>>>> /* trigger the operation */
>>>>>> io_uring_enter();
>>>>>>
>>>>>> I guess some kind of IORING_REGISTER_CREDS[_UPDATE] would be
>>>>>> good, together with a IOSQE_FIXED_CREDS in order to specify
>>>>>> credentials per operation.
>>>>>>
>>>>>> Or we make it much more generic and introduce a credsfd_create()
>>>>>> syscall in order to get an fd for a credential handle, maybe
>>>>>> together with another syscall to activate the credentials of
>>>>>> the current thread (or let a write to the fd trigger the activation
>>>>>> in order to avoid an additional syscall number).
>>>>>>
>>>>>> Having just an fd would allow IORING_REGISTER_CREDS[_UPDATE]
>>>>>> to be just an array of int values instead of a more complex
>>>>>> structure to define the credentials.
>>>>>
>>>>> I'd rather avoid having to add more infrastructure for this, even if
>>>>> credsfd_create() would be nifty.
>>>>>
>>>>> With that in mind, something like:
>>>>>
>>>>> - Application does IORING_REGISTER_CREDS, which returns some index
>>>>>
>>>>> - Add a IORING_OP_USE_CREDS opcode, which sets the creds associated
>>>>> with dependent commands
>>>>> - Actual request is linked to the IORING_OP_USE_CREDS command, any
>>>>> link off IORING_OP_USE_CREDS will use those credentials
>>>>
>>>> Using links for this sounds ok.
>>>
>>> Great! I'll try and hack this up and see how it turns out.
>>>
>>>>> - IORING_UNREGISTER_CREDS removes the registered creds
>>>>>
>>>>> Just throwing that out there, definitely willing to entertain other
>>>>> methods that make sense for this. Trying to avoid needing to put this
>>>>> information in the SQE itself, hence the idea to use a chain of links
>>>>> for it.
>>>>>
>>>>> The downside is that we'll need to maintain an array of key -> creds,
>>>>> but that's probably not a big deal.
>>>>>
>>>>> What do you think?
>>>>
>>>> So IORING_REGISTER_CREDS would be a simple operation that just takes a
>>>> snapshot of the current_cred() and returns an id that can be passed to
>>>> IORING_OP_USE_CREDS or IORING_UNREGISTER_CREDS?
>>>
>>> Right, you would not pass in any arguments, it'd have to be run from the
>>> personality you wish to register. It simply returns an integer, which is
>>> a key to use for IORING_OP_USE_CREDS, or at the end for
>>> IORING_UNREGISTER_CREDS when you no longer wish to use this personality.
>>>
>>>>> Ideally I'd like to get this done for 5.6 even if we
>>>>> are a bit late, so you'll have everything you need with that release.
>>>>
>>>> That would be great!
>>>
>>> Crossing fingers...
>>
>> OK, so here are two patches for testing:
>>
>> https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds
>>
>> #1 adds support for registering the personality of the invoking task,
>> and #2 adds support for IORING_OP_USE_CREDS. Right now it's limited to
>> just having one link, it doesn't support a chain of them.
>>
>> I'll try and write a test case for this just to see if it actually works,
>> so far it's totally untested.
>>
>> Adding Pavel to the CC.
>
> Minor tweak to ensuring we do the right thing for async offload as well,
> and it tests fine for me. Test case is:
>
> - Run as root
> - Register personality for root
> - create root only file
> - check we can IORING_OP_OPENAT the file
> - switch to user id test
> - check we cannot IORING_OP_OPENAT the file
> - check that we can open the file with IORING_OP_USE_CREDS linked

I didn't like it becoming a bit too complicated, both in terms of
implementation and use. And the fact that we'd have to jump through
hoops to make this work for a full chain.

So I punted and just added sqe->personality and IOSQE_PERSONALITY.
This makes it way easier to use. Same branch:

https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds

I'd feel much better with this variant for 5.6.

--
Jens Axboe