linux-kernel - Re: [PATCH 1/2] splice: export do

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Date:   Mon, 4 May 2020 15:31:07 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>
Cc:     io-uring <io-uring@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Clay Harris <bugs@...ycon.org>
Subject: Re: [PATCH 1/2] splice: export do_tee()

On 04/05/2020 14:09, Jann Horn wrote:
> On Sat, May 2, 2020 at 2:10 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>> export do_tee() for use in io_uring
> [...]
>> diff --git a/fs/splice.c b/fs/splice.c
> [...]
>>   * The 'flags' used are the SPLICE_F_* variants, currently the only
>>   * applicable one is SPLICE_F_NONBLOCK.
>>   */
>> -static long do_tee(struct file *in, struct file *out, size_t len,
>> -                  unsigned int flags)
>> +long do_tee(struct file *in, struct file *out, size_t len, unsigned int flags)
>>  {
>>         struct pipe_inode_info *ipipe = get_pipe_info(in);
>>         struct pipe_inode_info *opipe = get_pipe_info(out);
> 
> AFAICS do_tee() in its current form is not something you should be
> making available to anything else, because the file mode checks are
> performed in sys_tee() instead of in do_tee(). (And I don't see any
> check for file modes in your uring patch, but maybe I missed it?) If
> you want to make do_tee() available elsewhere, please refactor the
> file mode checks over into do_tee().

Overlooked it indeed. Glad you found it

> 
> The same thing seems to be true for the splice support, which luckily
> hasn't landed in a kernel release yet... while do_splice() does a
> random assortment of checks, the checks that actually consistently
> enforce the rules happen in sys_splice(). From a quick look,
> do_splice() doesn't seem to check:
> 
>  - when splicing from a pipe to a non-pipe: whether read access to the
> input pipe exists
>  - when splicing from a non-pipe to a pipe: whether write access to
> the output pipe exists
> 
> ... which AFAICS means that io_uring probably lets you get full R/W
> access to any pipe to which you're supposed to have either read or
> write access. (Although admittedly it is rare in practice that you get
> one end of a pipe and can't access the other one.)
> 
> When you expose previously internal helpers to io_uring, please have a
> look at their callers and see whether they perform any checks that
> look relevant.
> 

-- 
Pavel Begunkov