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[79.49.203.52]) by smtp.gmail.com with ESMTPSA id w16sm26837072wrg.95.2020.07.13.01.07.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2020 01:07:32 -0700 (PDT) Date: Mon, 13 Jul 2020 10:07:29 +0200 From: Stefano Garzarella To: Jens Axboe Cc: Sargun Dhillon , Kees Cook , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Kernel Hardening , Jann Horn , Aleksa Sarai , Christian Brauner , Stefan Hajnoczi , io-uring@vger.kernel.org, Alexander Viro , Jeff Moyer Subject: Re: [PATCH RFC 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS opcode Message-ID: <20200713080729.gttt3ymk7aqumle4@steredhat> References: <20200710141945.129329-1-sgarzare@redhat.com> <20200710141945.129329-3-sgarzare@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Fri, Jul 10, 2020 at 11:52:48AM -0600, Jens Axboe wrote: > On 7/10/20 8:19 AM, Stefano Garzarella wrote: > > The new io_uring_register(2) IOURING_REGISTER_RESTRICTIONS opcode > > permanently installs a feature whitelist on an io_ring_ctx. > > The io_ring_ctx can then be passed to untrusted code with the > > knowledge that only operations present in the whitelist can be > > executed. > > > > The whitelist approach ensures that new features added to io_uring > > do not accidentally become available when an existing application > > is launched on a newer kernel version. > > Keeping with the trend of the times, you should probably use 'allowlist' > here instead of 'whitelist'. Sure, it is better! > > > > Currently is it possible to restrict sqe opcodes and register > > opcodes. It is also possible to allow only fixed files. > > > > IOURING_REGISTER_RESTRICTIONS can only be made once. Afterwards > > it is not possible to change restrictions anymore. > > This prevents untrusted code from removing restrictions. > > A few comments below. > > > @@ -337,6 +344,7 @@ struct io_ring_ctx { > > struct llist_head file_put_llist; > > > > struct work_struct exit_work; > > + struct io_restriction restrictions; > > }; > > > > /* > > Since very few will use this feature, was going to suggest that we make > it dynamically allocated. But it's just 32 bytes, currently, so probably > not worth the effort... > Yeah, I'm not sure it will grow in the future, so I'm tempted to leave it as it is, but I can easily change it if you prefer. > > @@ -5491,6 +5499,11 @@ static int io_req_set_file(struct io_submit_state *state, struct io_kiocb *req, > > if (unlikely(!fixed && io_async_submit(req->ctx))) > > return -EBADF; > > > > + if (unlikely(!fixed && req->ctx->restrictions.enabled && > > + test_bit(IORING_RESTRICTION_FIXED_FILES_ONLY, > > + req->ctx->restrictions.restriction_op))) > > + return -EACCES; > > + > > return io_file_get(state, req, fd, &req->file, fixed); > > } > > This one hurts, though. I don't want any extra overhead from the > feature, and you're digging deep in ctx here to figure out of we need to > check. > > Generally, all the checking needs to be out-of-line, and it needs to > base the decision on whether to check something or not on a cache hot > piece of data. So I'd suggest to turn all of these into some flag. > ctx->flags generally mirrors setup flags, so probably just add a: > > unsigned int restrictions : 1; > > after eventfd_async : 1 in io_ring_ctx. That's free, plenty of room > there and that cacheline is already pulled in for reading. > Thanks for the clear explanation! I left a TODO comment near the 'enabled' field to look for something better, and what you're suggesting is what I was looking for :-) I'll change it! Thanks, Stefano