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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list\:FILESYSTEMS \(VFS and infrastructure\)" 
        <linux-fsdevel@vger.kernel.org>
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        <eb2da7e4-23ff-597a-08e1-e0555d490f6f@gmail.com>
Date:   Mon, 04 Nov 2019 17:41:40 -0600
In-Reply-To: <eb2da7e4-23ff-597a-08e1-e0555d490f6f@gmail.com> (Topi
        Miettinen's message of "Mon, 4 Nov 2019 19:58:55 +0200")
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Subject: Re: [PATCH] Allow restricting permissions in /proc/sys
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Topi Miettinen <toiwoton@gmail.com> writes:

> On 4.11.2019 17.44, Eric W. Biederman wrote:
>> Topi Miettinen <toiwoton@gmail.com> writes:
>>
>>> On 3.11.2019 20.50, Eric W. Biederman wrote:
>>>> Topi Miettinen <toiwoton@gmail.com> writes:
>>>>
>>>>> Several items in /proc/sys need not be accessible to unprivileged
>>>>> tasks. Let the system administrator change the permissions, but only
>>>>> to more restrictive modes than what the sysctl tables allow.
>>>>
>>>> This looks quite buggy.  You neither update table->mode nor
>>>> do you ever read from table->mode to initialize the inode.
>>>> I am missing something in my quick reading of your patch?
>>>
>>> inode->i_mode gets initialized in proc_sys_make_inode().
>>>
>>> I didn't want to touch the table, so that the original permissions can
>>> be used to restrict the changes made. In case the restrictions are
>>> removed as suggested by Theodore Ts'o, table->mode could be
>>> changed. Otherwise I'd rather add a new field to store the current
>>> mode and the mode field can remain for reference. As the original
>>> author of the code from 2007, would you let the administrator to
>>> chmod/chown the items in /proc/sys without restrictions (e.g. 0400 ->
>>> 0777)?
>>
>> At an architectural level I think we need to do this carefully and have
>> a compelling reason.  The code has survived nearly the entire life of
>> linux without this capability.
>
> I'd be happy with only allowing restrictions to access for
> now. Perhaps later with more analysis, also relaxing changes and maybe
> UID/GID changes can be allowed.

Let's find the use case where someone cares before we think about that.

>> I think right now the common solution is to mount another file over the
>> file you are trying to hide/limit.  Changing the permissions might be
>> better but that is not at all clear.
>>
>> Do you have specific examples of the cases where you would like to
>> change the permissions?
>
> Unprivileged applications typically do not need to access most items
> in /proc/sys, so I'd like to gradually find out which are needed. So
> far I've seen no problems with 0500 mode for directories abi, crypto,
> debug, dev, fs, user or vm.

But if there is no problem in letting everyone access the information
why reduce the permissions?

> I'm also using systemd's InaccessiblePaths to limit access (which
> mounts an inaccessible directory over the path), but that's a bit too
> big hammer. For example there are over 100 files in /proc/sys/kernel,
> perhaps there will be issues when creating a mount for each, and that
> multiplied by a number of services.

My sense is that if there is any kind of compelling reason to make
world-readable values not world-readable, and it doesn't break anything
(except malicious applications) than a kernel patch is probably the way
to go.

Policy knobs like this on proc tend to break in normal maintenance
because they are not used enough so I am not a big fan of adding policy
knobs just because we can.

> I see no problems by using Firejail (which uses PID namespacing) with
> v2, the permissions in /proc/sys are the same as outside the
> namespace.

Thank you for testing.

Eric