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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Tue, 26 Oct 2021 21:55:54 +0800
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Subject: Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to
 task comm size change
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
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On Tue, Oct 26, 2021 at 9:12 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 26 Oct 2021 10:18:51 +0800
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > > So, if we're ever going to copying these buffers out of the kernel (I
> > > don't know what the object lifetime here in bpf is for "e", etc), we
> > > should be zero-padding (as get_task_comm() does).
> > >
> > > Should this, instead, be using a bounce buffer?
> >
> > The comment in bpf_probe_read_kernel_str_common() says
> >
> >   :      /*
> >   :       * The strncpy_from_kernel_nofault() call will likely not fill the
> >   :       * entire buffer, but that's okay in this circumstance as we're probing
> >   :       * arbitrary memory anyway similar to bpf_probe_read_*() and might
> >   :       * as well probe the stack. Thus, memory is explicitly cleared
> >   :       * only in error case, so that improper users ignoring return
> >   :       * code altogether don't copy garbage; otherwise length of string
> >   :       * is returned that can be used for bpf_perf_event_output() et al.
> >   :       */
> >
> > It seems that it doesn't matter if the buffer is filled as that is
> > probing arbitrary memory.
> >
> > >
> > > get_task_comm(comm, task->group_leader);
> >
> > This helper can't be used by the BPF programs, as it is not exported to BPF.
> >
> > > bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm), comm);
>
> I guess Kees is worried that e.comm will have something exported to user
> space that it shouldn't. But since e is part of the BPF program, does the
> BPF JIT take care to make sure everything on its stack is zero'd out, such
> that a user BPF couldn't just read various items off its stack and by doing
> so, see kernel memory it shouldn't be seeing?
>

Understood.
It can leak information to the user if the user buffer is large enough.


> I'm guessing it does, otherwise this would be a bigger issue than this
> patch series.
>

I will think about how to fix it.
At first glance, it seems we'd better introduce a new BPF helper like
bpf_probe_read_kernel_str_pad().

-- 
Thanks
Yafang