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: <CAGXu5j+4g0YRkbZ7mq-3Zgc7aWCU-hx6+G_onW+DdK+mYcqPDA@mail.gmail.com>
Date:   Mon, 25 Feb 2019 08:48:34 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Sat, Feb 23, 2019 at 8:38 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Sat, Feb 23, 2019 at 4:44 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Sat, 23 Feb 2019 12:47:46 +0900
> > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > > Since kprobes handler runs in IRQ context, we can not use access_ok() in it.
> > > (only on x86 + CONFIG_DEBUG_ATOMIC_SLEEP=y)
> >
> > Is it really IRQ context or exception context? That is, one
> > (interrupts) happen for any task, but exceptions happen because of the
> > software that is executed (like a breakpoint). Although you can have a
> > kprobe trigger in an interrupt handler (where user access wouldn't make
> > sense anyway). But there should be no problem with user access from an
> > exception handler.
> >
>
> Can we just get rid of this might_sleep()?  access_ok() doesn't sleep
> as far as I know.

We do need to be aware of the userfaultfd case of getting held by
userspace in the middle of a copy_*_user()... that's a whole other
problem.

-- 
Kees Cook