Re: [PATCH 1/6] ebpf: add a seccomp program type

Alexei Starovoitov
2015-09-09T16:07:52Z

On Wed, Sep 09, 2015 at 09:50:35AM -0600, Tycho Andersen wrote:
> > >
> > > That's effectively what this patch does; when the eBPF is loaded via
> > > bpf(), you tell bpf() you want a BPF_PROG_TYPE_SECCOMP, and it invokes
> > > this validation/translation code, i.e. it uses
> > > seccomp_is_valid_access() to check and make sure access are aligned
> > > and inside struct seccomp_data.
> >
> > What about limiting the possible instructions?
>
> I totally overlooked this. A quick glance through the eBPF verifier
> makes me think that we can just add another function to struct
> bpf_verifier_ops called valid_instruction, which shouldn't be too
> hard. Perhaps a more interesting question is what to allow:
>
> BPF_LD(X) and BPF_ST(X): it looks like all types of stores are
> allowed, and only BPF_MEM and BPF_IMM loads are allowed; I think
> these can stay the same. BPF_XADD is new in eBPF, and I don't think
> we need it for seccomp (yet), since we don't have any shared memory
> via maps.
>
> BPF_ALU: It looks like we're also not allowing regular BPF_ALU
> instruction BPF_MOD; eBPF adds a few ones: BPF_MOV (register move),
> BPF_ARSH (sign extended right shift), and BPF_END (endianness
> conversion), wich I think should all be safe. In particular, we need
> to allow BPF_MOV at least, since that's how the converter implements
> BPF_MISC | BPF_TAX from classic.
>
> BPF_ALU64: I think we can safely allow all these as above, since
> they're just the 64-bit versions.
>
> BPF_JMP: eBPF adds BPF_JNE, BPF_JSGT, BPF_JSGE, BPF_CALL, and
> BPF_EXIT, which I think all should be safe (except maybe BPF_CALL
> since we're not allowing functions really). Again we have to allow
> one of the new eBPF codes, as the converter implements BPF_RET as
> BPF_JMP | BPF_EXIT.
>
> Thoughts?

Please do not add any per-instruction hacks. None of them are
necessary. Classic had to do extra ugly checks in seccomp only
because verifier wasn't flexible enough.
If you don't want to see any BPF_CALL in seccomp, just have
empty get_func_proto() callback for BPF_PROG_TYPE_SECCOMP
and verifier will reject all calls.
Currently we have only two non-generic instrucitons
LD_ABS and LD_IND that are avaialable for sockets/TC only,
because these are legacy instructions and we had to make
exceptions for them.