From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 277D4C2D0DB for ; Tue, 28 Jan 2020 21:27:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 02BC622522 for ; Tue, 28 Jan 2020 21:27:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726240AbgA1V1n (ORCPT ); Tue, 28 Jan 2020 16:27:43 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:38246 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726211AbgA1V1n (ORCPT ); Tue, 28 Jan 2020 16:27:43 -0500 Received: from ip5f5bd665.dynamic.kabel-deutschland.de ([95.91.214.101] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1iwYMY-0002dt-4j; Tue, 28 Jan 2020 21:25:34 +0000 Date: Tue, 28 Jan 2020 22:25:33 +0100 From: Christian Brauner To: Jens Axboe Cc: Pavel Begunkov , Stefan Metzmacher , io-uring , Linux API Mailing List Subject: Re: IORING_REGISTER_CREDS[_UPDATE]() and credfd_create()? Message-ID: <20200128212533.snjm34gct3kmfxfi@wittgenstein> References: <688e187a-75dd-89d9-921c-67de228605ce@samba.org> <1ac31828-e915-6180-cdb4-36685442ea75@kernel.dk> <0d4f43d8-a0c4-920b-5b8f-127c1c5a3fad@kernel.dk> <15ca72fd-5750-db7c-2404-2dd4d53dd196@gmail.com> <82b20ec2-ceaa-93f1-4cce-889a933f2c7a@kernel.dk> <60253bd9-93a7-4d76-93b6-586e4f55138c@gmail.com> <43a57f2a-16da-e657-3dca-5aa3afe31318@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <43a57f2a-16da-e657-3dca-5aa3afe31318@kernel.dk> Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Tue, Jan 28, 2020 at 01:56:00PM -0700, Jens Axboe wrote: > On 1/28/20 1:50 PM, Pavel Begunkov wrote: > > On 28/01/2020 23:19, Jens Axboe wrote: > >> On 1/28/20 1:16 PM, Pavel Begunkov wrote: > >>> On 28/01/2020 22:42, Jens Axboe wrote: > >>>> 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: > >>>>>> 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. > >>>> > >>> > >>> To be honest, sounds pretty dangerous. Especially since somebody started talking > >>> about stealing fds from a process, it could lead to a nasty loophole somehow. > >>> E.g. root registers its credentials, passes io_uring it to non-privileged > >>> children, and then some process steals the uring fd (though, it would need > >>> priviledged mode for code-injection or else). Could we Cc here someone really > >>> keen on security? > >> > >> Link? If you can steal fds, then surely you've already lost any sense of > > > > https://lwn.net/Articles/808997/ > > But I didn't looked up it yet. > > This isn't new by any stretch, it's always been possible to pass file > descriptors through SCM_RIGHTS. This just gives you a new way to do it. > That's not stealing or leaking, it's deliberately passing it to someone > else. I've been reading along quietly. In addition to what Jens said, to ease everyone's mind: pidfd_getfd() doesn't allow to unconditionally grab file descriptors for any task. That would be crazy. The calling task needs ptrace_may_access() permissions on the target task, i.e. the task from which you want to grab the io_uring file descriptor. And any calling task that has ptrace_may_access() permissions on the target can do much worse than just grabbing an fd. Christian