Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate" eBPF instruction

From: Andy Lutomirski
Date: Mon Aug 25 2014 - 21:54:56 EST

On Mon, Aug 25, 2014 at 6:53 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
> On Mon, Aug 25, 2014 at 6:38 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mon, Aug 25, 2014 at 6:35 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>>> On Mon, Aug 25, 2014 at 6:06 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>>>> From: Alexei Starovoitov <ast@xxxxxxxxxxxx>
>>>> Date: Mon, 25 Aug 2014 18:00:53 -0700
>>>>> add BPF_LD_IMM64 instruction to load 64-bit immediate value into a register.
>>>> I think you need to rethink this.
>>>> I understand that you want to be able to compile arbitrary C code into
>>>> eBPF, but you have to restrict strongly what data the eBPF code can get
>>>> to.
>>> I believe verifier already does restrict it. I don't see any holes in
>>> the architecture. I'm probably not explaining it clearly though :(
>>>> Arbitrary pointer loads is asking for trouble.
>>> Of course.
>>> There is no arbitrary pointer from user space.
>>> Verifier checks all pointers.
>>> I guess this commit log description is confusing.
>>> It says:
>>> BPF_LD_IMM64(R1, const_imm_map_ptr)
>>> that's what appears in the program _after_ it goes through verifier.
>>> User space cannot pass a pointer into the kernel.
>> If you don't intend for userspace to load a program that contains this
>> instruction, then why does it need to be an instruction that the
>> verifier rewrites? Why not have an instruction "load immediate
> user space use _pseudo_ bpf_ld_imm64 instruction.
> _pseudo_ stands for using 'map_fd' as imm instead of pointer.
>> relocated pointer" that contains a reference to a relocation table and
> Andy, I guess you missed explanation in:
> "
> Obviously user space doesn't know what kernel map pointer is associated
> with process-local map-FD.
> So it's using pseudo BPF_LD_IMM64 instruction.
> BPF_LD_IMM64 with src_reg == 0 -> generic move 64-bit immediate into dst_reg
> BPF_LD_IMM64 with src_reg == BPF_PSEUDO_MAP_FD -> mov map_fd into dst_reg
> Other values are reserved for now. (They will be used to implement
> global variables, strings and other constants and per-cpu areas in the future)
> So the programs look like:
> BPF_LD_MAP_FD(BPF_REG_1, process_local_map_fd),
> BPF_CALL(BPF_FUNC_map_lookup_elem),
> eBPF verifier scans the program for such pseudo instructions, converts
> process_local_map_fd -> in-kernel map pointer
> and drops 'pseudo' flag of BPF_LD_IMM64 instruction.
> "

Will a program that uses BPF_LD_IMM64 w/o the FPG_REG_1 thing be accepted?

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