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

Jens Axboe
2020-01-28T18:04:44Z

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

--
Jens Axboe