netdev - Re: [PATCH] bpf: verifier: BPF_MOV don't mark dst reg if src == dst

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Date:   Sun, 29 Jul 2018 18:59:02 +0300
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Arthur Fabre <afabre@...udflare.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH] bpf: verifier: BPF_MOV don't mark dst reg if src == dst

On Thu, Jul 26, 2018 at 1:08 AM, Arthur Fabre <afabre@...udflare.com> wrote:
> When check_alu_op() handles a BPF_MOV between two registers,
> it calls check_reg_arg() on the dst register, marking it as unbounded.
> If the src and dst register are the same, this marks the src as
> unbounded, which can lead to unexpected errors for further checks that
> rely on bounds info.
>
> check_alu_op() now only marks the dst register as unbounded if it
> different from the src register.
>
> Signed-off-by: Arthur Fabre <afabre@...udflare.com>
> ---
>  kernel/bpf/verifier.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 63aaac52a265..ddfe3c544a80 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3238,8 +3238,9 @@ static int check_alu_op(struct bpf_verifier_env
> *env, struct bpf_insn *insn)
>                         }
>                 }
>
> -               /* check dest operand */
> -               err = check_reg_arg(env, insn->dst_reg, DST_OP);
> +               /* check dest operand, only mark if dest != src */
> +               err = check_reg_arg(env, insn->dst_reg,
> +                               insn->dst_reg == insn->src_reg ?
> DST_OP_NO_MARK : DST_OP);

that doesn't look correct for 32-bit mov.
Is that the case you're trying to improve?