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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Tue, 26 Oct 2021 22:02:36 +0800
Message-ID: <CALOAHbAa-iMD4k2DEOun+RivUXiSMKR6ndCsqGZMseUbX_9+ww@mail.gmail.com>
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>,
        Petr Mladek <pmladek@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Qiang Zhang <qiang.zhang@windriver.com>,
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        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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On Tue, Oct 26, 2021 at 9:55 PM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> 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?
> >
>

Ah, you mean the BPF JIT has already avoided leaking information to user.
I will check the BPF JIT code first.

> 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


-- 
Thanks
Yafang