Re: [PATCH v2 bpf-next 1/3] bpf: Support for pointers beyond pkt_end.

Daniel Borkmann
2020-11-12T23:56:58Z

On 11/12/20 8:16 PM, John Fastabend wrote:
> Alexei Starovoitov wrote:
>> From: Alexei Starovoitov <ast@kernel.org>
>>
>> This patch adds the verifier support to recognize inlined branch conditions.
>> The LLVM knows that the branch evaluates to the same value, but the verifier
>> couldn't track it. Hence causing valid programs to be rejected.
>> The potential LLVM workaround: https://reviews.llvm.org/D87428
>> can have undesired side effects, since LLVM doesn't know that
>> skb->data/data_end are being compared. LLVM has to introduce extra boolean
>> variable and use inline_asm trick to force easier for the verifier assembly.
>>
>> Instead teach the verifier to recognize that
>> r1 = skb->data;
>> r1 += 10;
>> r2 = skb->data_end;
>> if (r1 > r2) {
>> here r1 points beyond packet_end and
>> subsequent
>> if (r1 > r2) // always evaluates to "true".
>> }
>>
>> Tested-by: Jiri Olsa <jolsa@redhat.com>
>> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>> ---
>> include/linux/bpf_verifier.h | 2 +-
>> kernel/bpf/verifier.c | 129 +++++++++++++++++++++++++++++------
>> 2 files changed, 108 insertions(+), 23 deletions(-)
>>
>
> Thanks, we can remove another set of inline asm logic.
>
> Acked-by: John Fastabend <john.fastabend@gmail.com>
>
>> if (pred >= 0) {
>> @@ -7517,7 +7601,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
>> */
>> if (!__is_pointer_value(false, dst_reg))
>> err = mark_chain_precision(env, insn->dst_reg);
>> - if (BPF_SRC(insn->code) == BPF_X && !err)
>> + if (BPF_SRC(insn->code) == BPF_X && !err &&
>> + !__is_pointer_value(false, src_reg))
>
> This could have been more specific with !type_is_pkt_pointer() correct? I
> think its fine as is though.
>
>> err = mark_chain_precision(env, insn->src_reg);
>> if (err)
>> return err;

Given the reg->range could now be negative, I wonder whether for the regsafe()
pruning logic we should now better add a >=0 sanity check in there before we
attempt to test on rold->range > rcur->range?

Thanks,
Daniel