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. > > > - 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. > > > - 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. > > > - 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. Another thing I'm concerned with is that the generated code is different, which introduces additional complication during debugging. > > 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. That can't be the reason to maintain that inconsistency. -- 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
