On 25-Feb 16:30, Casey Schaufler wrote: > On 2/24/2020 9:41 PM, Alexei Starovoitov wrote: > > On Mon, Feb 24, 2020 at 01:41:19PM -0800, Kees Cook wrote: > >> But the LSM subsystem doesn't want special cases (Casey has worked very > >> hard to generalize everything there for stacking). It is really hard to > >> accept adding a new special case when there are still special cases yet > >> to be worked out even in the LSM code itself[2]. > >> [2] Casey's work to generalize the LSM interfaces continues and it quite > >> complex: > >> https://lore.kernel.org/linux-security-module/20200214234203.7086-1-casey@xxxxxxxxxxxxxxxx/ > > I think the key mistake we made is that we classified KRSI as LSM. > > LSM stacking, lsmblobs that the above set is trying to do are not necessary for KRSI. > > I don't see anything in LSM infra that KRSI can reuse. > > The only thing BPF needs is a function to attach to. > > It can be a nop function or any other. > > security_*() functions are interesting from that angle only. > > Hence I propose to reconsider what I was suggesting earlier. > > No changes to secruity/ directory. > > Attach to security_*() funcs via bpf trampoline. > > The key observation vs what I was saying earlier is KRSI and LSM are wrong names. > > I think "security" is also loaded word that should be avoided. > > No argument there. > > > I'm proposing to rename BPF_PROG_TYPE_LSM into BPF_PROG_TYPE_OVERRIDE_RETURN. > > > >> So, unless James is going to take this over Casey's objections, the path > >> forward I see here is: > >> > >> - land a "slow" KRSI (i.e. one that hooks every hook with a stub). > >> - optimize calling for all LSMs > > I'm very much surprised how 'slow' KRSI is an option at all. > > 'slow' KRSI means that CONFIG_SECURITY_KRSI=y adds indirect calls to nop > > functions for every place in the kernel that calls security_*(). > > This is not an acceptable overhead. Even w/o retpoline > > this is not something datacenter servers can use. > > In the universe I live in data centers will disable hyper-threading, > reducing performance substantially, in the face of hypothetical security > exploits. That's a massively greater performance impact than the handful > of instructions required to do indirect calls. Not to mention the impact Indirect calls have worse performance implications than just a few instructions and are especially not suitable for hotpaths. There have been multiple efforts to reduce their usage e.g.: - https://lwn.net/Articles/774743/ - https://lwn.net/Articles/773985/ > of the BPF programs that have been included. Have you ever looked at what BPF programs are JIT'ed and optimized to native code. > happens to system performance when polkitd is enabled? However, let's discuss all this separately when we follow-up with performance improvements after submitting the initial patch-set. > > > > > > Another option is to do this: > > diff --git a/include/linux/security.h b/include/linux/security.h > > index 64b19f050343..7887ce636fb1 100644 > > --- a/include/linux/security.h > > +++ b/include/linux/security.h > > @@ -240,7 +240,7 @@ static inline const char *kernel_load_data_id_str(enum kernel_load_data_id id) > > return kernel_load_data_str[id]; > > } > > > > -#ifdef CONFIG_SECURITY > > +#if defined(CONFIG_SECURITY) || defined(CONFIG_BPF_OVERRIDE_RETURN) > > > > Single line change to security.h and new file kernel/bpf/override_security.c > > that will look like: > > int security_binder_set_context_mgr(struct task_struct *mgr) > > { > > return 0; > > } > > > > int security_binder_transaction(struct task_struct *from, > > struct task_struct *to) > > { > > return 0; > > } > > Essentially it will provide BPF side with a set of nop functions. > > CONFIG_SECURITY is off. It may seem as a downside that it will force a choice > > on kernel users. Either they build the kernel with CONFIG_SECURITY and their > > choice of LSMs or build the kernel with CONFIG_BPF_OVERRIDE_RETURN and use > > BPF_PROG_TYPE_OVERRIDE_RETURN programs to enforce any kind of policy. I think > > it's a pro not a con. > > Err, no. All distros use an LSM or two. Unless you can re-implement SELinux The users mentioned here in this context are (I would assume) the more performance sensitive users who would, potentially, disable CONFIG_SECURITY because of the current performance characteristics. We can also discuss this separately and only if we find that we need it for the BPF_OVERRIDE_RET type attachment. - KP > in BPF (good luck with state transitions) you've built a warp drive without > ever having mined dilithium crystals. > >
