()On Tue, Feb 11, 2020 at 9:10 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > On Tue, Feb 11, 2020 at 08:36:18PM +0100, Jann Horn wrote: > > On Tue, Feb 11, 2020 at 8:09 PM Alexei Starovoitov > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > On Tue, Feb 11, 2020 at 07:44:05PM +0100, Jann Horn wrote: > > > > On Tue, Feb 11, 2020 at 6:58 PM Alexei Starovoitov > > > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > On Tue, Feb 11, 2020 at 01:43:34PM +0100, KP Singh wrote: > > > > [...] > > > > > > * When using the semantic provided by fexit, the BPF LSM program will > > > > > > always be executed and will be able to override / clobber the > > > > > > decision of LSMs which appear before it in the ordered list. This > > > > > > semantic is very different from what we currently have (i.e. the BPF > > > > > > LSM hook is only called if all the other LSMs allow the action) and > > > > > > seems to be bypassing the LSM framework. > > > > > > > > > > It that's a concern it's trivial to add 'if (RC == 0)' check to fexit > > > > > trampoline generator specific to lsm progs. > > > > [...] > > > > > Using fexit mechanism and bpf_sk_storage generalization is > > > > > all that is needed. None of it should touch security/*. > > > > > > > > If I understand your suggestion correctly, that seems like a terrible > > > > idea to me from the perspective of inspectability and debuggability. > > > > If at runtime, a function can branch off elsewhere to modify its > > > > decision, I want to see that in the source code. If someone e.g. > > > > changes the parameters or the locking rules around a security hook, > > > > how are they supposed to understand the implications if that happens > > > > through some magic fexit trampoline that is injected at runtime? > > > > > > I'm not following the concern. There is error injection facility that is > > > heavily used with and without bpf. In this case there is really no difference > > > whether trampoline is used with direct call or indirect callback via function > > > pointer. Both will jump to bpf prog. The _source code_ of bpf program will > > > _always_ be available for humans to examine via "bpftool prog dump" since BTF > > > is required. So from inspectability and debuggability point of view lsm+bpf > > > stuff is way more visible than any builtin LSM. At any time people will be able > > > to see what exactly is running on the system. Assuming folks can read C code. > > > > You said that you want to use fexit without touching security/, which > > AFAIU means that the branch from security_*() to the BPF LSM will be > > invisible in the *kernel's* source code unless the reader already > > knows about the BPF LSM. But maybe I'm just misunderstanding your > > idea. > > > > If a random developer is trying to change the locking rules around > > security_blah(), and wants to e.g. figure out whether it's okay to > > call that thing with a spinlock held, or whether one of the arguments > > is actually used, or stuff like that, the obvious way to verify that > > is to follow all the direct and indirect calls made from > > security_blah(). It's tedious, but it works, unless something is > > hooked up to it in a way that is visible in no way in the source code. > > > > I agree that the way in which the call happens behind the scenes > > doesn't matter all that much - I don't really care all that much > > whether it's an indirect call, a runtime-patched direct call in inline > > assembly, or an fexit hook. What I do care about is that someone > > reading through any affected function can immediately see that the > > branch exists - in other words, ideally, I'd like it to be something > > happening in the method body, but if you think that's unacceptable, I > > think there should at least be a function attribute that makes it very > > clear what's going on. > > Got it. Then let's whitelist them ? > All error injection points are marked with ALLOW_ERROR_INJECTION(). > We can do something similar here, but let's do it via BTF and avoid > abusing yet another elf section for this mark. > I think BTF_TYPE_EMIT() should work. Just need to pick explicit enough > name and extensive comment about what is going on. Sounds reasonable to me. :) > Locking rules and cleanup around security_blah() shouldn't change though. > Like security_task_alloc() should be paired with security_task_free(). > And so on. With bpf_sk_storage like logic the alloc/free of scratch > space will be similar to the way socket and bpf progs deal with it. > > Some of the lsm hooks are in critical path. Like security_socket_sendmsg(). > retpoline hurts. If we go with indirect calls right now it will be harder to > optimize later. It took us long time to come up with bpf trampoline and build > bpf dispatcher on top of it to remove single indirect call from XDP runtime. > For bpf+lsm would be good to avoid it from the start. Just out of curiosity: Are fexit hooks really much cheaper than indirect calls? AFAIK ftrace on x86-64 replaces the return pointer for fexit instrumentation (see prepare_ftrace_return()). So when the function returns, there is one return misprediction for branching into return_to_handler(), and then the processor's internal return stack will probably be misaligned so that after ftrace_return_to_handler() is done running, all the following returns will also be mispredicted. So I would've thought that fexit hooks would have at least roughly the same impact as indirect calls - indirect calls via retpoline do one mispredicted branch, fexit hooks do at least two AFAICS. But I guess indirect calls could still be slower if fexit benefits from having all the mispredicted pointers stored on the cache-hot stack while the indirect branch target is too infrequently accessed to be in L1D, or something like that?
