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

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Date:   Fri, 24 Jan 2020 09:58:01 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Stefan Metzmacher <metze@...ba.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, io-uring <io-uring@...r.kernel.org>
Subject: Re: [PATCH 5.4 033/222] io_uring: only allow submit from owning task

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@...nel.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@...r.kernel.org
>> Suggested-by: Stefan Metzmacher <metze@...ba.org>
>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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.

-- 
Jens Axboe