netdev - Re: Bug with BPF_ALU64

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Date:   Thu, 14 Sep 2017 14:22:55 -0700
From:   Y Song <ys114321@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Edward Cree <ecree@...arflare.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Bug with BPF_ALU64 | BPF_END?

On Thu, Sep 14, 2017 at 11:14 AM, David Miller <davem@...emloft.net> wrote:
> From: Edward Cree <ecree@...arflare.com>
> Date: Thu, 14 Sep 2017 18:53:17 +0100
>
>> Is BPF_END supposed to only be used with BPF_ALU, never with BPF_ALU64?

Yes, only BPF_ALU. The below is LLVM bpf swap insn encoding:

...
// bswap16, bswap32, bswap64
class BSWAP ...
...
  let op = 0xd;     // BPF_END
  let BPFSrc = 1;   // BPF_TO_BE (TODO: use BPF_TO_LE for big-endian target)
  let BPFClass = 4; // BPF_ALU
...

>> In kernel/bpf/core.c:___bpf_prog_run(), there are only jump table targets
>>  for the BPF_ALU case, not for the BPF_ALU64 case (opcodes 0xd7 and 0xdf).
>> But the verifier doesn't enforce this; by crafting a program that uses
>>  these opcodes I can get a WARN when they're run (without JIT; it looks
>>  like the x86 JIT, at least, won't like it either).
>> Proposed patch below the cut; build-tested only.
>
> Good catch.
>
> A really neat test would be a program that uploads random BPF programs
> into the kernel, in a syzkaller'ish way.  It might have triggered this
> (eventually).
>