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From:   Daurnimator <quae@daurnimator.com>
Date:   Wed, 22 Jul 2020 12:35:15 +1000
Message-ID: <CAEnbY+fCP-HS_rWfOF2rnUPos-eZRF1dL+m2Q8CZidi_W=a7xw@mail.gmail.com>
Subject: Re: [PATCH RFC v2 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS opcode
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Stefano Garzarella <sgarzare@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Kees Cook <keescook@chromium.org>,
        Aleksa Sarai <asarai@suse.de>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Jann Horn <jannh@google.com>,
        io-uring <io-uring@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>,
        linux-kernel@vger.kernel.org
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On Wed, 22 Jul 2020 at 03:11, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 7/21/20 4:40 AM, Stefano Garzarella wrote:
> > On Thu, Jul 16, 2020 at 03:26:51PM -0600, Jens Axboe wrote:
> >> On 7/16/20 6:48 AM, Stefano Garzarella wrote:
> >>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> >>> index efc50bd0af34..0774d5382c65 100644
> >>> --- a/include/uapi/linux/io_uring.h
> >>> +++ b/include/uapi/linux/io_uring.h
> >>> @@ -265,6 +265,7 @@ enum {
> >>>     IORING_REGISTER_PROBE,
> >>>     IORING_REGISTER_PERSONALITY,
> >>>     IORING_UNREGISTER_PERSONALITY,
> >>> +   IORING_REGISTER_RESTRICTIONS,
> >>>
> >>>     /* this goes last */
> >>>     IORING_REGISTER_LAST
> >>> @@ -293,4 +294,30 @@ struct io_uring_probe {
> >>>     struct io_uring_probe_op ops[0];
> >>>  };
> >>>
> >>> +struct io_uring_restriction {
> >>> +   __u16 opcode;
> >>> +   union {
> >>> +           __u8 register_op; /* IORING_RESTRICTION_REGISTER_OP */
> >>> +           __u8 sqe_op;      /* IORING_RESTRICTION_SQE_OP */
> >>> +   };
> >>> +   __u8 resv;
> >>> +   __u32 resv2[3];
> >>> +};
> >>> +
> >>> +/*
> >>> + * io_uring_restriction->opcode values
> >>> + */
> >>> +enum {
> >>> +   /* Allow an io_uring_register(2) opcode */
> >>> +   IORING_RESTRICTION_REGISTER_OP,
> >>> +
> >>> +   /* Allow an sqe opcode */
> >>> +   IORING_RESTRICTION_SQE_OP,
> >>> +
> >>> +   /* Only allow fixed files */
> >>> +   IORING_RESTRICTION_FIXED_FILES_ONLY,
> >>> +
> >>> +   IORING_RESTRICTION_LAST
> >>> +};
> >>> +
> >>
> >> Not sure I totally love this API. Maybe it'd be cleaner to have separate
> >> ops for this, instead of muxing it like this. One for registering op
> >> code restrictions, and one for disallowing other parts (like fixed
> >> files, etc).
> >>
> >> I think that would look a lot cleaner than the above.
> >>
> >
> > Talking with Stefan, an alternative, maybe more near to your suggestion,
> > would be to remove the 'struct io_uring_restriction' and add the
> > following register ops:
> >
> >     /* Allow an sqe opcode */
> >     IORING_REGISTER_RESTRICTION_SQE_OP
> >
> >     /* Allow an io_uring_register(2) opcode */
> >     IORING_REGISTER_RESTRICTION_REG_OP
> >
> >     /* Register IORING_RESTRICTION_*  */
> >     IORING_REGISTER_RESTRICTION_OP
> >
> >
> >     enum {
> >         /* Only allow fixed files */
> >         IORING_RESTRICTION_FIXED_FILES_ONLY,
> >
> >         IORING_RESTRICTION_LAST
> >     }
> >
> >
> > We can also enable restriction only when the rings started, to avoid to
> > register IORING_REGISTER_ENABLE_RINGS opcode. Once rings are started,
> > the restrictions cannot be changed or disabled.
>
> My concerns are largely:
>
> 1) An API that's straight forward to use
> 2) Something that'll work with future changes
>
> The "allow these opcodes" is straightforward, and ditto for the register
> opcodes. The fixed file I guess is the odd one out. So if we need to
> disallow things in the future, we'll need to add a new restriction
> sub-op. Should this perhaps be "these flags must be set", and that could
> easily be augmented with "these flags must not be set"?
>
> --
> Jens Axboe
>

This is starting to sound a lot like seccomp filtering.
Perhaps we should go straight to adding a BPF hook that fires when
reading off the submission queue?