netdev - Re: [PATCH net] bpf: don't let ldimm64 leak map addresses on unprivileged

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Message-ID: <CAG48ez0S92fLmX5fEa1=HNC3wF+SYizske5MjiJDpW4=tLg6Uw@mail.gmail.com>
Date:   Mon, 8 May 2017 00:26:10 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kafai@...com, netdev@...r.kernel.org
Subject: Re: [PATCH net] bpf: don't let ldimm64 leak map addresses on unprivileged

On Mon, May 8, 2017 at 12:04 AM, Daniel Borkmann <daniel@...earbox.net> wrote:
> The patch fixes two things at once:
>
> 1) It checks the env->allow_ptr_leaks and only prints the map address to
>    the log if we have the privileges to do so, otherwise it just dumps 0
>    as we would when kptr_restrict is enabled on %pK. Given the latter is
>    off by default and not every distro sets it, I don't want to rely on
>    this, hence the 0 by default for unprivileged.
>
> 2) Printing of ldimm64 in the verifier log is currently broken in that
>    we don't print the full immediate, but only the 32 bit part of the
>    first insn part for ldimm64. Thus, fix this up as well; it's okay to
>    access, since we verified all ldimm64 earlier already (including just
>    constants) through replace_map_fd_with_map_ptr().
>
> Fixes: cbd357008604 ("bpf: verifier (add ability to receive verification log)")
> Reported-by: Jann Horn <jannh@...gle.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
[...]
> @@ -362,9 +363,19 @@ static void print_bpf_insn(struct bpf_insn *insn)
>                                 insn->code,
>                                 bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
>                                 insn->src_reg, insn->imm);
> -               } else if (BPF_MODE(insn->code) == BPF_IMM) {
> -                       verbose("(%02x) r%d = 0x%x\n",
> -                               insn->code, insn->dst_reg, insn->imm);
> +               } else if (BPF_MODE(insn->code) == BPF_IMM &&
> +                          BPF_SIZE(insn->code) == BPF_DW) {
> +                       /* At this point, we already made sure that the second
> +                        * part of the ldimm64 insn is accessible.
> +                        */
> +                       u64 imm = ((u64)(insn + 1)->imm << 32) | (u32)insn->imm;
> +                       bool map_ptr = insn->src_reg == BPF_PSEUDO_MAP_FD;
> +
> +                       if (map_ptr && !env->allow_ptr_leaks)
> +                               imm = 0;
> +
> +                       verbose("(%02x) r%d = 0x%llx\n", insn->code,
> +                               insn->dst_reg, (unsigned long long)imm);
>                 } else {
>                         verbose("BUG_ld_%02x\n", insn->code);
>                         return;

You replaced the `BPF_MODE(insn->code) == BPF_IMM` branch with a
`BPF_MODE(insn->code) == BPF_IMM && BPF_SIZE(insn->code) == BPF_DW`
branch. Doesn't that break printing normal immediates?