linux-kernel - Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

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Date:   Wed, 21 Feb 2018 10:24:05 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set
 out of range

On 02/20/2018 06:17 PM, Andrew Morton wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long <longman@...hat.com> wrote:
>
>> Even with clamped sysctl parameters, it is still not that straight
>> forward to figure out the exact range of those parameters. One may
>> try to write extreme parameter values to see if they get clamped.
>> To make it easier, a warning with the expected range will now be
>> printed in the kernel ring buffer when a clamped sysctl parameter
>> receives an out of range value.
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
>
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?
>
Good point. Beside adding security check, another alternative is to use
some kind of warn_once() for each sysctl parameter. That will limit the
amount of spamming that is possible. It will require adding a flag to
the ctl_table to mark an  entry as warned. I think that will be less
tricky than adding permission check.

I can also use the new flag to designate that a sysctl parameter need to
be clamped to the range instead of failing when out of range. With that,
I don't need to introduce new clamping APIs. I can use the existing
ones. I will work on v2 patch with that change.

Thanks,
Longman