Re: [PATCH bpf-next v2 0/3] bpf: add boot parameters for sysctl knobs

[Alexei Starovoitov]

On Fri, May 25, 2018 at 06:50:09PM +0200, Eugene Syromiatnikov wrote:
> On Thu, May 24, 2018 at 04:34:51PM -0700, Alexei Starovoitov wrote:
> > On Thu, May 24, 2018 at 09:41:08AM +0200, Jesper Dangaard Brouer wrote:
> > > On Wed, 23 May 2018 15:02:45 -0700
> > > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> > > 
> > > > On Wed, May 23, 2018 at 02:18:19PM +0200, Eugene Syromiatnikov wrote:
> > > > > Some BPF sysctl knobs affect the loading of BPF programs, and during
> > > > > system boot/init stages these sysctls are not yet configured.
> > > > > A concrete example is systemd, that has implemented loading of BPF
> > > > > programs.
> > > > > 
> > > > > Thus, to allow controlling these setting at early boot, this patch set
> > > > > adds the ability to change the default setting of these sysctl knobs
> > > > > as well as option to override them via a boot-time kernel parameter
> > > > > (in order to avoid rebuilding kernel each time a need of changing these
> > > > > defaults arises).
> > > > > 
> > > > > The sysctl knobs in question are kernel.unprivileged_bpf_disable,
> > > > > net.core.bpf_jit_harden, and net.core.bpf_jit_kallsyms.  
> > > > 
> > > > - systemd is root. today it only uses cgroup-bpf progs which require root,
> > > >   so disabling unpriv during boot time makes no difference to systemd.
> > > >   what is the actual reason to present time?
> systemd also runs a lot of code, some of which is unprivileged.

systemd processes sysctl configs first. It's essential for system
security to do so. If you have concerns in how systemd does that
please bring it up with systemd folks.

> > > > - say in the future systemd wants to use so_reuseport+bpf for faster
> > > >   networking. With unpriv disable during boot, it will force systemd
> > > >   to do such networking from root, which will lower its security barrier.
> No, it will force systemd not to use SO_REUSEPORT BPF.

sorry this argument makes no sense to me.

> > > > - bpf_jit_kallsyms sysctl has immediate effect on loaded programs.
> > > >   Flipping it during the boot or right after or any time after
> > > >   is the same thing. Why add such boot flag then?
> Well, that one was for completeness.

Should we convert all sysctls to bootparams for 'completeness' ?

> > > > - jit_harden can be turned on by systemd. so turning it during the boot
> > > >   will make systemd progs to be constant blinded.
> > > >   Constant blinding protects kernel from unprivileged JIT spraying.
> > > >   Are you worried that systemd will attack the kernel with JIT spraying?
> I'm worried that systemd can be exploited for a JIT spraying attack.

I'm afraid we're not on the same page with definition of 'JIT spraying attack'.

> Another thing I'm concerned with is that the generated code is different,
> which introduces additional complication during debugging.

specifically what kind of complication?