linux-kernel - Re: [PATCH] Allow restricting permissions in /proc/sys

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Date:   Mon, 04 Nov 2019 09:44:05 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list\:FILESYSTEMS \(VFS and infrastructure\)" 
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys

Topi Miettinen <toiwoton@...il.com> writes:

> On 3.11.2019 20.50, Eric W. Biederman wrote:
>> Topi Miettinen <toiwoton@...il.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 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?

>> The not updating table->mode almost certainly means that as soon as the
>> cached inode is invalidated the mode changes will disappear.  Not to
>> mention they will fail to propogate between  different instances of
>> proc.
>>
>> Loosing all of your changes at cache invalidation seems to make this a
>> useless feature.
>
> At least different proc instances seem to work just fine here (they
> show the same changes), but I suppose you are right about cache
> invalidation.

It is going to take the creation of a pid namespace to see different
proc instances.  All mounts of the proc within the same pid_namespace
return the same instance.

Eric