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Date: Fri, 10 Jul 2020 11:52:48 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Sargun Dhillon <sargun@...gun.me>, Kees Cook <keescook@...omium.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Jann Horn <jannh@...gle.com>, Aleksa Sarai <asarai@...e.de>,
Christian Brauner <christian.brauner@...ntu.com>,
Stefan Hajnoczi <stefanha@...hat.com>, io-uring@...r.kernel.org,
Alexander Viro <viro@...iv.linux.org.uk>, Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH RFC 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS
opcode
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
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