linux-hardening - Re: Per-process flag set via prctl() to deny module loading?

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Message-ID: <ZDR+4R4AKfnBcMOu@tycho.pizza>
Date:   Mon, 10 Apr 2023 15:25:53 -0600
From:   Tycho Andersen <tycho@...ho.pizza>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     linux-modules <linux-modules@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Per-process flag set via prctl() to deny module loading?

On Mon, Apr 10, 2023 at 11:47:16PM +0300, Topi Miettinen wrote:
> On 10.4.2023 16.36, Tycho Andersen wrote:
> > On Mon, Apr 10, 2023 at 01:06:00PM +0300, Topi Miettinen wrote:
> > > I'd propose to add a per-process flag to irrevocably deny any loading of
> > > kernel modules for the process and its children. The flag could be set (but
> > > not unset) via prctl() and for unprivileged processes, only when
> > > NoNewPrivileges is also set. This would be similar to CAP_SYS_MODULE, but
> > > unlike capabilities, there would be no issues with namespaces since the flag
> > > isn't namespaced.
> > > 
> > > The implementation should be very simple.
> > > 
> > > Preferably the flag, when configured, would be set by systemd, Firejail and
> > > maybe also container managers. The expectation would be that the permission
> > > to load modules would be retained only by udev and where SUID needs to be
> > > allowed (NoNewPrivileges unset).
> > 
> > You can do something like this today via STATIC_USERMODEHELPER without
> > the need for kernel patches. It is a bit heavyweight for a
> > general-purpose system though.
> 
> So the user mode helper would be launched whenever there is a module request
> and it would check whether the process is allowed to load modules or not?

Yes, exactly.

> Does it know which process caused the module to be loaded and what were its
> credentials at that time?

It doesn't know which process caused the module load, which is kind of
unfortunate. It looks like you could stick it in the environment in
kernel/kmod.c:call_modprobe() without breaking too many things,
though.

Tycho