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

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Message-ID: <CAOn4ftuW+8DcdHTEP3CYwRjSEOfeWfoxZ=eUp8aTuLYmrNADXQ@mail.gmail.com>
Date:   Mon, 30 Jul 2018 08:44:09 +0100
From:   Arthur Fabre <afabre@...udflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 Sun, Jul 29, 2018 at 4:59 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> 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?

The patch was originally for 64-bit mov only