Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Steve,

On Wed, 20 Feb 2019 09:49:26 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 20 Feb 2019 17:10:19 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > Let me ensure what you want. So you want to access a "string" in user-space,
> > not a data structure? In that case, it is very easy to me. It is enough to
> > add a "ustring" type to kprobe events. For example, do_sys_opsn's path 
> > variable is one example. That will be +0(+0(%si)):ustring, and fetcher
> > finally copy the string using strncpy_from_user() instead of 
> > strncpy_from_unsafe(). (*)
> 
> ustring would be good.

I've tried to implement ustring and u-offsets, but I got some issues.

- access_ok() warns if it is called in IRQ context (kprobes is.)
- copy_from_user uses access_ok(), so it is not designed for irq handler.

Moreover, if we have different kernel/user address spaces, we have to
assign target user-pages to kernel vma. Can we do that (doesn't it involve
mutex locks)?

If not, I think what we can do "in kprobes" is only probe_kernel_read()
and strncpy_from_unsafe(). This means, on the architechture whoes kernel
address space doesn't map user space, we can not support user-space
data fetching in kprobe evevnts.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>