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Subject: Re: [PATCH RFC v2 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS
 opcode
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     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@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>,
        linux-kernel@vger.kernel.org
References: <20200716124833.93667-1-sgarzare@redhat.com>
 <20200716124833.93667-3-sgarzare@redhat.com>
 <0fbb0393-c14f-3576-26b1-8bb22d2e0615@kernel.dk>
 <20200721104009.lg626hmls5y6ihdr@steredhat>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <15f7fcf5-c5bb-7752-fa9a-376c4c7fc147@kernel.dk>
Date:   Tue, 21 Jul 2020 11:11:17 -0600
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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