netdev - Re: More BPF verifier questions

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Date:   Mon, 5 Jun 2017 00:06:41 -0700
From:   Y Song <ys114321@...il.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>
Subject: Re: More BPF verifier questions

On Fri, Jun 2, 2017 at 7:42 AM, Edward Cree <ecree@...arflare.com> wrote:
> A couple of the tests in tools/testing/selftests/bpf/test_verifier.c seem to be bogus: Test "multiple registers share map_lookup_elem bad reg type" is supposed to
>  error with "R3 invalid mem access 'inv'", but from my reading of it, R3 gets
>  loaded with a map_value_or_null, that later gets null-checked (both directly
>  and - through R0 - indirectly), and finally stored through.  I don't see
>  what's supposed to make R3 become a bad pointer.

You are right. In this case,
r0 = bpf_map_lookup
r2 = r0
r3 = r0
r4 = r0
r5 = r0
if (r0 != 0)  <=== condition 1
  r1 = 1
if (r0 != 0)
  r1 = 2
if (r3 != 0)
  *r3 = 0
...

If (r0 != 0) if false, the current verifier marks r2/r3/r4/r5 as unknown value.
I guess here what you did to have precise value 0 helps and make verifier
complaint go away correctly.

> Test "helper access to variable memory: stack, bitwise AND + JMP, correct
>  bounds" is listed as expected to pass, but it passes zero in the 'size'
>  argument, an ARG_CONST_SIZE, to bpf_probe_read; I believe this should fail
>  (and with my WIP patch it does).

Probably a typo or mis-statement. "size" is not passed in with "zero", but
with an unknown value. Hence, it probably should fail.

      BPF_MOV64_IMM(BPF_REG_2, 16),
      BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2, -128),
      BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1, -128),
      BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 64),
      BPF_MOV64_IMM(BPF_REG_4, 0),
      BPF_JMP_REG(BPF_JGE, BPF_REG_4, BPF_REG_2, 2),
      BPF_MOV64_IMM(BPF_REG_3, 0),
      BPF_EMIT_CALL(BPF_FUNC_probe_read),


In kernel/bpf/verifier.c,

  } else if (arg_type == ARG_CONST_SIZE ||
       arg_type == ARG_CONST_SIZE_OR_ZERO) {
    expected_type = CONST_IMM;
    /* One exception. Allow UNKNOWN_VALUE registers when the
     * boundaries are known and don't cause unsafe memory accesses
     */
    if (type != UNKNOWN_VALUE && type != expected_type)
      goto err_type;

Maybe somebody can provide some historical context for this relaxation.