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

[Jann Horn]

On Wed, Feb 20, 2019 at 9:10 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> On Tue, 19 Feb 2019 14:03:30 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > > > Basically, a kprobe is mostly used for debugging what's happening in a
> > > > live kernel, to read any address.
> > >
> > > My point is that "any address" is not sufficient to begin with. You
> > > need "kernel or user".
> > >
> > > Having a flag for what _kind_ of kernel address is ok might then be
> > > required for other cases if they might not be ok with following page
> > > tables to IO space..
> > >
> >
> > Good point. Looks like we should add a new flag for kprobe
> > trace parameters, that tell kprobes if the address is expected to be
> > user or kernel. That would be good regardless of the duplicate
> > meanings, as we could use copy_from_user without touching KERNEL_DS, if
> > the probe argument specifically states "this is user space". For
> > example, when probing do_sys_open, and you want to read what path string
> > was passed into the kernel.
> >
> > Masami, thoughts?
>
> 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(). (*)
[...]
> (*) BTW, there is another concern to use _from_user APIs in kprobe. Are those
> APIs might sleep??

If you want to access userspace without sleeping, and ignore data in
non-present pages, you can do `pagefault_disable(); err =
__copy_from_user_inatomic(...); pagefault_enable();`. (Actually, maybe
the kernel should have a helper for that...)