Re: [PATCH 5.4 033/222] io_uring: only allow submit from owning task

[Stefan Metzmacher <metze@samba.org>]

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Am 24.01.20 um 17:58 schrieb Jens Axboe:
> On 1/24/20 3:38 AM, Stefan Metzmacher wrote:
>> Am 22.01.20 um 10:26 schrieb Greg Kroah-Hartman:
>>> From: Jens Axboe <axboe@kernel.dk>
>>>
>>> commit 44d282796f81eb1debc1d7cb53245b4cb3214cb5 upstream.
>>>
>>> If the credentials or the mm doesn't match, don't allow the task to
>>> submit anything on behalf of this ring. The task that owns the ring can
>>> pass the file descriptor to another task, but we don't want to allow
>>> that task to submit an SQE that then assumes the ring mm and creds if
>>> it needs to go async.
>>>
>>> Cc: stable@vger.kernel.org
>>> Suggested-by: Stefan Metzmacher <metze@samba.org>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>
>>>
>>> ---
>>>  fs/io_uring.c |    6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -3716,6 +3716,12 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned
>>>  			wake_up(&ctx->sqo_wait);
>>>  		submitted = to_submit;
>>>  	} else if (to_submit) {
>>> +		if (current->mm != ctx->sqo_mm ||
>>> +		    current_cred() != ctx->creds) {
>>> +			ret = -EPERM;
>>> +			goto out;
>>> +		}
>>> +
>>
>> I thought about this a bit more.
>>
>> I'm not sure if this is actually to restrictive,
>> because it means applications like Samba won't
>> be able to use io-uring at all.
>>
>> As even if current_cred() and ctx->creds describe the same
>> set of uid,gids the != won't ever match again and
>> makes the whole ring unuseable.
>>
>> I'm not sure about what the best short term solution could be...
>>
>> 1. May just doing the check for path based operations?
>>   and fail individual requests with EPERM.
>>
>> 2. Or force REQ_F_FORCE_ASYNC for path based operations,
>>   so that they're always executed from within the workqueue
>>   with were ctx->creds is active.
>>
>> 3. Or (as proposed earlier) do the override_creds/revert_creds dance
>>   (and similar for mm) if needed.
>>
>> To summaries the problem again:
>>
>> For path based operations like:
>> - IORING_OP_CONNECT (maybe also - IORING_OP_ACCEPT???)
>> - IORING_OP_SEND*, IORING_OP_RECV* on DGRAM sockets
>> - IORING_OP_OPENAT, IORING_OP_STATX, IORING_OP_OPENAT2
>> it's important under which current_cred they are called.
>>
>> Are IORING_OP_MADVISE, IORING_OP_FADVISE and IORING_OP_FALLOCATE
>> are only bound to the credentials of the passed fd they operate on?
>>
>> The current assumption is that the io_uring_setup() syscall captures
>> the current_cred() to ctx->cred and all operations on the ring
>> are executed under the context of ctx->cred.
>> Therefore all helper threads do the override_creds/revert_creds dance.
> 
> But it doesn't - we're expecting them to match, and with this change,
> we assert that it's the case or return -EPERM.
> 
>> But the possible non-blocking line execution of operations in
>> the io_uring_enter() syscall doesn't do the override_creds/revert_creds
>> dance and execute the operations under current_cred().
>>
>> This means it's random depending on filled cached under what
>> credentials an operation is executed.
>>
>> In order to prevent security problems the current patch is enough,
>> but as outlined above it will make io-uring complete unuseable
>> for applications using any syscall that changes current_cred().
>>
>> Change 1. would be a little bit better, but still not really useful.
>>
>> I'd actually prefer solution 3. as it's still possible to make
>> use of non-blocking operations, while the security is the
>> same as solution 2.
> 
> For your situation, we need to extend it anyway, and provide a way
> to swap between personalities. So yeah it won't work as-is for your
> use case, but we can work on making that the case.

That's only for the OPENAT2 case, which we might want to use in future,
but there's a lot of work required to have async opens in Samba.

But I have a experimental module that, just use READV, WRITEV and FSYNC
with io-uring in order to avoid our userspace helper threads.

And that won't work anymore with the change as Samba change
current_cred() very often switch between (at least) 2 identities
root and the user. That will change the pointer of current_cred() each time.

I mean I could work around the check by using IORING_SETUP_SQPOLL,
but I'd like to avoid that.

metze


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