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

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Message-ID: <CAHk-=whX6LLVn_q2DC0K37iQgZ4S0LX6mR46+O2TNufJuR-K6Q@mail.gmail.com>
Date:   Fri, 22 Feb 2019 10:28:49 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Fri, Feb 22, 2019 at 9:48 AM Andy Lutomirski <luto@...capital.net> wrote:
>
> > On Feb 22, 2019, at 9:43 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >
> > Then we should still probably fix up "__probe_kernel_read()" to not
> > allow user accesses. The easiest way to do that is actually likely to
> > use the "unsafe_get_user()" functions *without* doing a
> > uaccess_begin(), which will mean that modern CPU's will simply fault
> > on a kernel access to user space.
> >
> > The nice thing about that is that usually developers will have access
> > to exactly those modern boxes, so the people who notice that it
> > doesn't work are the right people.
>
> We use probe_kernel_read() from oops code. I’d rather it return -EFAULT than oops harder and kill the first oops.

It would still do that.

Using the unsafe_get_user() macros doesn't remove the exception
handling, and we wouldn't remove the whole "pagefault_disable()"
either. So it would work exactly the same way it does now, except on a
modern CPU it would return -EFAULT for a user space access due to AC
not being set.

So no "oops harder", only "safer accesses".

                Linus