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? > > > > - 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. > > How that make sense? > > > > - 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? > > > > - 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 think you are missing that, we want the ability to change these > defaults in-order to avoid depending on /etc/sysctl.conf settings, and > that the these sysctl.conf setting happen too late. What does it mean 'happens too late' ? Too late for what? sysctl.conf has plenty of system critical knobs like kernel.perf_event_paranoid, kernel.core_pattern, etc The behavior of the host is drastically different after sysctl config is applied. > For example with jit_harden, there will be a difference between the > loaded BPF program that got loaded at boot-time with systemd (no > constant blinding) and when someone reloads that systemd service after > /etc/sysctl.conf have been evaluated and setting bpf_jit_harden (now > slower due to constant blinding). This is inconsistent behavior. net.core.bpf_jit_harden can be flipped back and forth at run-time, so bpf progs before and after will be either blinded or not. I don't see any inconsistency. In general I think bootparams should be used only for things like kpti=on/off that cannot be set by sysctl. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
