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References: <74a91362-247c-c749-5200-7bdce704ed9e@gmail.com> <20191112232239.yevpeemgxz4wy32b@wittgenstein>
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From:   Jann Horn <jannh@google.com>
Date:   Wed, 13 Nov 2019 17:00:19 +0100
Message-ID: <CAG48ez0j_7NyCyvGn8U8NS2p=CQQb=me-5KTa7k5E6xpHJphaA@mail.gmail.com>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys
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>,
        Linux API <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, Nov 13, 2019 at 12:22 AM Christian Brauner
<christian.brauner@ubuntu.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.
> >
> > Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
> > ---
> >  fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
> >  1 file changed, 31 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index d80989b6c344..88c4ca7d2782 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int
> > mask)
> >         if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
> >                 return -EACCES;
> >
> > +       error = generic_permission(inode, mask);
> > +       if (error)
> > +               return error;

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?