netdev - Re: [PATCH v3 bpf-next 06/11] bpf: implement accurate raw

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Date:   Wed, 16 Oct 2019 23:52:02 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 06/11] bpf: implement accurate raw_tp context
 access via BTF

On 10/16/19 3:08 PM, Daniel Borkmann wrote:
> On 10/16/19 11:28 PM, Alexei Starovoitov wrote:
>> On Wed, Oct 16, 2019 at 2:22 PM Daniel Borkmann <daniel@...earbox.net> 
>> wrote:
>>> On 10/16/19 5:25 AM, Alexei Starovoitov wrote:
>>>> libbpf analyzes bpf C program, searches in-kernel BTF for given type 
>>>> name
>>>> and stores it into expected_attach_type.
>>>> The kernel verifier expects this btf_id to point to something like:
>>>> typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, 
>>>> void *loc);
>>>> which represents signature of raw_tracepoint "kfree_skb".
>>>>
>>>> Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb'
>>>> and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint.
>>>> In first case it passes btf_id of 'struct sk_buff *' back to the 
>>>> verifier core
>>>> and 'void *' in second case.
>>>>
>>>> Then the verifier tracks PTR_TO_BTF_ID as any other pointer type.
>>>> Like PTR_TO_SOCKET points to 'struct bpf_sock',
>>>> PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on.
>>>> PTR_TO_BTF_ID points to in-kernel structs.
>>>> If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF
>>>> then PTR_TO_BTF_ID#1234 points to one of in kernel skbs.
>>>>
>>>> When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32)
>>>> the btf_struct_access() checks which field of 'struct sk_buff' is
>>>> at offset 32. Checks that size of access matches type definition
>>>> of the field and continues to track the dereferenced type.
>>>> If that field was a pointer to 'struct net_device' the r2's type
>>>> will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device'
>>>> in vmlinux's BTF.
>>>>
>>>> Such verifier analysis prevents "cheating" in BPF C program.
>>>> The program cannot cast arbitrary pointer to 'struct sk_buff *'
>>>> and access it. C compiler would allow type cast, of course,
>>>> but the verifier will notice type mismatch based on BPF assembly
>>>> and in-kernel BTF.
>>>>
>>>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>>
>>> Overall set looks great!
>>>
>>> [...]
>>>> +int btf_struct_access(struct bpf_verifier_log *log,
>>>> +                   const struct btf_type *t, int off, int size,
>>>> +                   enum bpf_access_type atype,
>>>> +                   u32 *next_btf_id)
>>>> +{
>>>> +     const struct btf_member *member;
>>>> +     const struct btf_type *mtype;
>>>> +     const char *tname, *mname;
>>>> +     int i, moff = 0, msize;
>>>> +
>>>> +again:
>>>> +     tname = __btf_name_by_offset(btf_vmlinux, t->name_off);
>>>
>>> More of a high-level question wrt btf_ctx_access(), is there a reason 
>>> the ctx
>>> access is only done for raw_tp? I presume kprobes is still on todo 
>>> (?), what
>>> about uprobes which also have pt_regs and could benefit from this 
>>> work, but is
>>> not fixed to btf_vmlinux to search its ctx type.
>>
>> Optimized kprobes via ftrace entry point are on immediate todo list
>> to follow up. I'm still debating on the best way to handle it.
>> uprobes - I haven't though about. Likely necessary as well.
>> Not sure what types to give to pt_regs yet.
>>
>>> I presume BPF_LDX | BPF_PROBE_MEM | BPF_* would need no additional 
>>> encoding,
>>> but JIT emission would have to differ depending on the prog type.
>>
>> you mean for kprobes/uprobes? Why would it need to be different?
>> The idea was to keep LDX|PROBE_MEM as normal LDX|MEM load as much as 
>> possible.
> 
> Agree, makes sense.
> 
>> The only difference vs normal load is to populate extable which is
>> arch dependent.
> 
> Wouldn't you also need to switch to USER_DS similarly to what 
> probe_kernel_read()
> vs probe_user_read() differentiates?

No. I don't think we should.
Here we're reading only kernel memory and shouldn't be
messing with addr_limit.
No stac/clac and no access_ok() either.
It's kernel memory being read.
set_fs(KERNEL_DS) matters when access_ok() and getuser()
are used by callee that normally take user address
while caller is passing kernel address.
Here is no such thing.