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

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Message-ID: <CAG48ez1qtVXSBCmLCWdRwfemw6u5d-Zargm-MNJV_N0WAAoVwg@mail.gmail.com>
Date:   Wed, 13 Nov 2019 17:40:33 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys

On Wed, Nov 13, 2019 at 5:19 PM Topi Miettinen <toiwoton@...il.com> wrote:
> On 13.11.2019 18.00, Jann Horn wrote:
> > On Wed, Nov 13, 2019 at 12:22 AM Christian Brauner
> > <christian.brauner@...ntu.com> wrote:
> >> On Sun, Nov 03, 2019 at 04:55:48PM +0200, Topi Miettinen wrote:
> >>> 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.
[...]
> > In kernel/ucount.c, the ->permissions handler set_permissions() grants
> > access based on whether the caller has CAP_SYS_RESOURCE. And in
> > net/sysctl_net.c, the handler net_ctl_permissions() grants access
> > based on whether the caller has CAP_NET_ADMIN. This added check is
> > going to break those, right?
> >
>
> Right. The comment above seems then a bit misleading:
>         /*
>          * sysctl entries that are not writeable,
>          * are _NOT_ writeable, capabilities or not.
>          */

I don't see the problem. Those handlers never make a file writable
that doesn't have one of the three write bits (0222) set.