On Tue, Jun 16, 2020 at 08:36:28PM +0200, Jann Horn wrote: > On Tue, Jun 16, 2020 at 5:49 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Tue, Jun 16, 2020 at 02:14:47PM +0200, Jann Horn wrote: > > > Wouldn't it be simpler to use a function that can run a subset of > > > seccomp cBPF and bails out on anything that indicates that a syscall's > > > handling is complex or on instructions it doesn't understand? For > > > syscalls that have a fixed policy, a typical seccomp filter doesn't > > > even use any of the BPF_ALU ops, the scratch space, or the X register; > > > it just uses something like the following set of operations, which is > > > easy to emulate without much code: > > > > > > BPF_LD | BPF_W | BPF_ABS > > > BPF_JMP | BPF_JEQ | BPF_K > > > BPF_JMP | BPF_JGE | BPF_K > > > BPF_JMP | BPF_JGT | BPF_K > > > BPF_JMP | BPF_JA > > > BPF_RET | BPF_K > > > > Initially, I started down this path. It needed a bit of plumbing into > > BPF to better control the lifetime of the cBPF "saved original filter" > > (normally used by CHECKPOINT_RESTORE uses) > > I don't think you need that? When a filter is added, you can compute > the results of the added individual filter, and then merge the state. That's what I thought too, but unfortunately not (unless I missed something) -- the seccomp verifier is run as a callback from the BPF internals, so seccomp only see what the user sends (which is unverified) and the final eBPF filter. There isn't state I can attach during the callback, so I opted to just do the same thing as CHECKPOINT_RESTORE, but to then explicitly free the cBPF after bitmap generation. > > and then I needed to keep > > making exceptions (same list you have: ALU, X register, scratch, etc) > > in the name of avoiding too much complexity in the emulator. I decided > > I'd rather reuse the existing infrastructure to actually execute the > > filter (no cBPF copy needed to be saved, no separate code, and full > > instruction coverage). > > If you really think that this bit of emulation is so bad, you could > also make a copy of the BPF filter in which you replace all load > instructions from syscall arguments with "return NON_CONSTANT_RESULT", > and then run that through the normal BPF infrastructure. > > > > Something like (completely untested): > [...] > > I didn't actually finish going down the emulator path (I stopped right > > around the time I verified that libseccomp does use BPF_ALU -- though > > only BPF_AND), so I didn't actually evaluate the filter contents for other > > filter builders (i.e. Chrome). > > > > But, if BPF_ALU | BPF_AND were added to your code above, it would cover > > everything libseccomp generates (which covers a lot of the seccomp > > filters, e.g. systemd, docker). I just felt funny about an "incomplete" > > emulator. > > > > Though now you've got me looking. It seems this is the core > > of Chrome's BPF instruction generation: > > https://github.com/chromium/chromium/blob/master/sandbox/linux/bpf_dsl/policy_compiler.cc > > It also uses ALU|AND, but adds JMP|JSET. > > > > So... that's only 2 more instructions to cover what I think are likely > > the two largest seccomp instruction generators. > > > > > That way, you won't need any of this complicated architecture-specific stuff. > > > > There are two arch-specific needs, and using a cBPF-subset emulator > > just gets rid of the local TLB flush. The other part is distinguishing > > the archs. Neither requirement is onerous (TLB flush usually just > > needs little more than an extern, arch is already documented in the > > per-arch syscall_get_arch()). > > But it's also somewhat layer-breaking and reliant on very specific > assumptions. Normal kernel code doesn't mess around with page table > magic, outside of very specific low-level things. And your method > would break if the fixed-value members were not all packed together at > the start of the structure. Right -- that was lucky. I suspect the emulation route will win out here. > And from a hardening perspective: The more code we add that fiddles > around with PTEs directly, rather than going through higher-level > abstractions, the higher the chance that something gets horribly > screwed up. For example, this bit from your patch looks *really* > suspect: > > + preempt_disable(); > + set_pte_at(&init_mm, vaddr, ptep, > pte_mkold(*(READ_ONCE(ptep)))); > + local_flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE); > + preempt_enable(); > > First off, that set_pte_at() is just a memory write; I don't see why > you put it inside a preempt_disable() region. > But more importantly, sticking a local TLB flush inside a > preempt_disable() region with nothing else in there looks really > shady. How is that supposed to work? If we migrate from CPU0 to CPU1 > directly before this region, and then from CPU1 back to CPU0 directly > afterwards, the local TLB flush will have no effect. Yeah, true, that's another good reason not to do this. -- Kees Cook _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
