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

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Date:   Thu, 26 Jul 2018 02:42:28 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Arthur Fabre <afabre@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH] bpf: verifier: BPF_MOV don't mark dst reg if src == dst

On 07/26/2018 12:08 AM, Arthur Fabre 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);
>                 if (err)
>                         return err;
> 

Thanks a lot for the patch! Looks like it's corrupted wrt newline.

Please also add test cases to tools/testing/selftests/bpf/test_verifier.c
for the cases of mov64 and mov32 where in each src==dst and src!=dst; mov32
should mark it as unbounded but not former, so would be good to keep tracking
that in selftests.