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-=wgTuK3kAduP-gr10vykT1uG=B2VpdffvmyBuTQ1UxPpMg@mail.gmail.com>
Date:   Tue, 19 Feb 2019 10:43:36 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Tue, Feb 19, 2019 at 8:18 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> > So it would be good to not just say "user or kernel", but actually say
> > what *kind* of kernel access it expects.
>
> Note, kprobes are a different kind of beast. I've used kprobes to probe
> userspace information as well as kernel. Heck, I could see someone
> even using kprobes to probe IO memory to check if a device is doing
> what they expect it's doing.

Note that even if that is the case, you _need_ to special "user vs
kernel" information.

Because the exact same address might exist in both.

Right now I think that only happens on sparc32, but vendors used to
have that issue on x86-32 too (if they had the 4G:4G patches).

> 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..

          Linus