netdev - Re: [PATCH bpf-next v2] bpf: reject kfunc calls that overflow insn->imm

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Date:   Wed, 9 Feb 2022 07:31:06 -0800
From:   Yonghong Song <yhs@...com>
To:     Hou Tao <houtao1@...wei.com>, Alexei Starovoitov <ast@...nel.org>
CC:     Martin KaFai Lau <kafai@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf: reject kfunc calls that overflow
 insn->imm



On 2/8/22 10:20 PM, Hou Tao wrote:
> Hi,
> 
> On 2/9/2022 12:57 AM, Yonghong Song wrote:
>>
>>
>> On 2/8/22 4:33 AM, Hou Tao wrote:
>>> Now kfunc call uses s32 to represent the offset between the address
>>> of kfunc and __bpf_call_base, but it doesn't check whether or not
>>> s32 will be overflowed, so add an extra checking to reject these
>>> invalid kfunc calls.
>>>
>>> Signed-off-by: Hou Tao <houtao1@...wei.com>
>>> ---
>>> v2:
>>>    * instead of checking the overflow in selftests, just reject
>>>      these kfunc calls directly in verifier
>>>
>>> v1: https://lore.kernel.org/bpf/20220206043107.18549-1-houtao1@huawei.com
>>> ---
>>>    kernel/bpf/verifier.c | 13 +++++++++++++
>>>    1 file changed, 13 insertions(+)
>>>
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index a39eedecc93a..fd836e64b701 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -1832,6 +1832,13 @@ static struct btf *find_kfunc_desc_btf(struct
>>> bpf_verifier_env *env,
>>>        return btf_vmlinux ?: ERR_PTR(-ENOENT);
>>>    }
>>>    +static inline bool is_kfunc_call_imm_overflowed(unsigned long addr)
>>> +{
>>> +    unsigned long offset = BPF_CALL_IMM(addr);
>>> +
>>> +    return (unsigned long)(s32)offset != offset;
>>> +}
>>> +
>>>    static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16
>>> offset)
>>>    {
>>>        const struct btf_type *func, *func_proto;
>>> @@ -1925,6 +1932,12 @@ static int add_kfunc_call(struct bpf_verifier_env
>>> *env, u32 func_id, s16 offset)
>>>            return -EINVAL;
>>>        }
>>>    +    if (is_kfunc_call_imm_overflowed(addr)) {
>>> +        verbose(env, "address of kernel function %s is out of range\n",
>>> +            func_name);
>>> +        return -EINVAL;
>>> +    }
>>> +
>>>        desc = &tab->descs[tab->nr_descs++];
>>>        desc->func_id = func_id;
>>>        desc->imm = BPF_CALL_IMM(addr);
>>
>> Thanks, I would like to call BPF_CALL_IMM only once and keep checking overflow
>> and setting desc->imm close to each other. How about the following
>> not-compile-tested code
>>
>>      unsigned long call_imm;
>>
>>      ...
>>      call_imm = BPF_CALL_IMM(addr);
>>      /* some comment here */
>>      if ((unsigned long)(s32)call_imm != call_imm) {
>>          verbose(env, ...);
>>          return -EINVAL;
>>      } else {
>>          desc->imm = call_imm;
>>      }
> call BPF_CALL_IMM once is OK for me. but I don't think the else branch is
> unnecessary and it make the code
> ugly. Can we just return directly when found that imm is overflowed ?
> 
>          call_imm = BPF_CALL_IMM(addr);
>          /* Check whether or not the relative offset overflows desc->imm */
>          if ((unsigned long)(s32)call_imm != call_imm) {
>                  verbose(env, "address of kernel function %s is out of range\n",
>                          func_name);
>                  return -EINVAL;
>          }
> 
>          desc = &tab->descs[tab->nr_descs++];
>          desc->func_id = func_id;
>          desc->imm = call_imm;

Sure. Your above change looks good. My change is just
an illustration :-).

> 
> 
> 
> 
>> .
> 
>