Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to task comm size change

[Steven Rostedt]

On Tue, 26 Oct 2021 10:18:51 +0800
Yafang Shao <laoar.shao@xxxxxxxxx> 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?

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

-- Steve