netdev - Re: [PATCH v4 bpf-next 1/2] bpf: introduce helper bpf_find

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <4C63F03A-0561-430F-949E-FE65D69CE222@fb.com>
Date:   Fri, 5 Nov 2021 22:52:42 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <ast@...com>
CC:     Hengqi Chen <hengqi.chen@...il.com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "Kernel Team" <Kernel-team@...com>, KP Singh <kpsingh@...nel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v4 bpf-next 1/2] bpf: introduce helper bpf_find_vma



> On Nov 5, 2021, at 2:16 PM, Alexei Starovoitov <ast@...com> wrote:
> 
> On 11/5/21 2:11 PM, Song Liu wrote:
>>> On Nov 5, 2021, at 8:23 AM, Hengqi Chen <hengqi.chen@...il.com> wrote:
>>> 
>>> Hi, Song
>>> 
>>> On 2021/11/5 5:31 AM, Song Liu wrote:
>>>> In some profiler use cases, it is necessary to map an address to the
>>>> backing file, e.g., a shared library. bpf_find_vma helper provides a
>>>> flexible way to achieve this. bpf_find_vma maps an address of a task to
>>>> the vma (vm_area_struct) for this address, and feed the vma to an callback
>>>> BPF function. The callback function is necessary here, as we need to
>>>> ensure mmap_sem is unlocked.
>>>> 
>>>> It is necessary to lock mmap_sem for find_vma. To lock and unlock mmap_sem
>>>> safely when irqs are disable, we use the same mechanism as stackmap with
>>>> build_id. Specifically, when irqs are disabled, the unlocked is postponed
>>>> in an irq_work. Refactor stackmap.c so that the irq_work is shared among
>>>> bpf_find_vma and stackmap helpers.
>>>> 
>>>> Reported-by: kernel test robot <lkp@...el.com>
>>>> Signed-off-by: Song Liu <songliubraving@...com>
>>>> ---
>>> 
>>> [...]
>>> 
>>>> 
>>>> -BTF_ID_LIST(btf_task_file_ids)
>>>> -BTF_ID(struct, file)
>>>> -BTF_ID(struct, vm_area_struct)
>>>> -
>>>> static const struct bpf_iter_seq_info task_seq_info = {
>>>> 	.seq_ops		= &task_seq_ops,
>>>> 	.init_seq_private	= init_seq_pidns,
>>>> @@ -586,9 +583,74 @@ static struct bpf_iter_reg task_vma_reg_info = {
>>>> 	.seq_info		= &task_vma_seq_info,
>>>> };
>>>> 
>>>> +BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
>>>> +	   bpf_callback_t, callback_fn, void *, callback_ctx, u64, flags)
>>>> +{
>>>> +	struct mmap_unlock_irq_work *work = NULL;
>>>> +	struct vm_area_struct *vma;
>>>> +	bool irq_work_busy = false;
>>>> +	struct mm_struct *mm;
>>>> +	int ret = -ENOENT;
>>>> +
>>>> +	if (flags)
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (!task)
>>>> +		return -ENOENT;
>>>> +
>>>> +	mm = task->mm;
>>>> +	if (!mm)
>>>> +		return -ENOENT;
>>>> +
>>>> +	irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
>>>> +
>>>> +	if (irq_work_busy || !mmap_read_trylock(mm))
>>>> +		return -EBUSY;
>>>> +
>>>> +	vma = find_vma(mm, start);
>>>> +
>>> 
>>> I found that when a BPF program attach to security_file_open which is in
>>> the bpf_d_path helper's allowlist, the bpf_d_path helper is also allowed
>>> to be called inside the callback function. So we can have this in callback
>>> function:
>>> 
>>>    bpf_d_path(&vma->vm_file->f_path, path, sizeof(path));
>>> 
>>> 
>>> I wonder whether there is a guarantee that vma->vm_file will never be null,
>>> as you said in the commit message, a backing file.
>> I don't think we can guarantee vma->vm_file never be NULL here, so this is
>> a real problem. Let me see how to fix it.
> 
> It's unrelated. There was a separate thread about this.

I see. I will not fix it here then. 

Thanks,
Song