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

[Masami Hiramatsu]

Hi Jann,

On Wed, 20 Feb 2019 14:57:31 +0100
Jann Horn <jannh@xxxxxxxxxx> wrote:

> 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...)

Ok, we are going back to the start point of this thread :)

http://lkml.kernel.org/r/20190215174712.372898450@xxxxxxxxxxx

So, if user tells kprobe it is user-pointer, we check it with access_ok(),
and will do something similar to the strnlen_user() and strncpy_from_user(),
but using __copy_from_user_inatomic() and pagefault_disable() for kprobes.

Thank you!

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>