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

[Topi Miettinen]

On 13.11.2019 18.00, Jann Horn wrote:
> On Wed, Nov 13, 2019 at 12:22 AM Christian Brauner
> <christian.brauner@xxxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >   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?

Right. The comment above seems then a bit misleading:
	/*
	 * sysctl entries that are not writeable,
	 * are _NOT_ writeable, capabilities or not.
	 */

-Topi