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From: Jens Axboe <axboe@kernel.dk>
To: Stefano Garzarella <sgarzare@redhat.com>
Cc: Sargun Dhillon <sargun@sargun.me>,
	Kees Cook <keescook@chromium.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Jann Horn <jannh@google.com>, Aleksa Sarai <asarai@suse.de>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	io-uring@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [PATCH RFC 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS opcode
Date: Fri, 10 Jul 2020 11:52:48 -0600	[thread overview]
Message-ID: <f39fe84d-1353-1066-c7fc-770054f7129e@kernel.dk> (raw)
In-Reply-To: <20200710141945.129329-3-sgarzare@redhat.com>

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'.
> 
> 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...

> @@ -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.


-- 
Jens Axboe


  reply	other threads:[~2020-07-10 17:52 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-10 14:19 [PATCH RFC 0/3] io_uring: add restrictions to support untrusted applications and guests Stefano Garzarella
2020-07-10 14:19 ` [PATCH RFC 1/3] io_uring: use an enumeration for io_uring_register(2) opcodes Stefano Garzarella
2020-07-10 14:19 ` [PATCH RFC 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS opcode Stefano Garzarella
2020-07-10 17:52   ` Jens Axboe [this message]
2020-07-13  8:07     ` Stefano Garzarella
2020-07-10 14:19 ` [PATCH RFC 3/3] io_uring: allow disabling rings during the creation Stefano Garzarella
2020-07-10 15:33 ` [PATCH RFC 0/3] io_uring: add restrictions to support untrusted applications and guests Konrad Rzeszutek Wilk
2020-07-10 16:20   ` Stefano Garzarella
2020-07-13  9:24     ` Stefan Hajnoczi

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