linux-kernel - Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event

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Date:   Mon, 1 Nov 2021 16:01:02 +0100
From:   Florent Revest <revest@...omium.org>
To:     Hengqi Chen <hengqi.chen@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kpsingh@...nel.org, jackmanb@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event_mmap

On Mon, Nov 1, 2021 at 2:17 PM Hengqi Chen <hengqi.chen@...il.com> wrote:
>
> Hi,
>
>
> On 2021/10/30 1:02 AM, Florent Revest wrote:
> > On Fri, Oct 29, 2021 at 12:47 AM Martin KaFai Lau <kafai@...com> wrote:
> >>
> >> On Thu, Oct 28, 2021 at 06:43:57PM +0200, Florent Revest wrote:
> >>> Allow the helper to be called from the perf_event_mmap hook. This is
> >>> convenient to lookup vma->vm_file and implement a similar logic as
> >>> perf_event_mmap_event in BPF.
> >> From struct vm_area_struct:
> >>         struct file * vm_file;          /* File we map to (can be NULL). */
> >>
> >> Under perf_event_mmap, vm_file won't be NULL or bpf_d_path can handle it?
> >
> > Thanks Martin, this is a very good point. :) Yes, vm_file can be NULL
> > in perf_event_mmap.
> > I wonder what would happen (and what we could do about it? :|).
> > bpf_d_path is called on &vma->vm_file->f_path So without NULL checks
> > (of vm_file) in BPF, the helper wouldn't be called with a NULL pointer
> > but rather with an address that is offsetof(struct file, f_path).
> >
>
> I tested this patch with the following BCC script:
>
>     bpf_text = '''
>     #include <linux/mm_types.h>
>
>     KFUNC_PROBE(perf_event_mmap, struct vm_area_struct *vma)
>     {
>         char path[256] = {};
>
>         bpf_d_path(&vma->vm_file->f_path, path, sizeof(path));
>         bpf_trace_printk("perf_event_mmap %s", path);
>         return 0;
>     }
>     '''
>
>     b = BPF(text=bpf_text)
>     print("BPF program loaded")
>     b.trace_print()
>
> This change causes kernel panic. I think it's because of this NULL pointer.

Thank you for the testing and repro Hengqi :)
Indeed, I was able to reproduce this panic. When vma->vm_file is NULL,
&vma->vm_file->f_path ends up being 0x18 so d_path causes a panic.
I suppose that this sort of issue must be relatively common in helpers
that take a PTR_TO_BTF_ID though ? I wonder if there is anything that
the verifier could do about this ? For example if vma->vm_file could
be PTR_TO_BTF_ID_OR_NULL and therefore vma->vm_file->f_path somehow
considered invalid ?