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

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Date:   Mon, 25 Feb 2019 00:17:57 +0900
From:   Masami Hiramatsu <mhiramat@...nel.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>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Sat, 23 Feb 2019 20:38:03 -0800
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.

Hmm, which might_sleep() would you pointed? What I talked was a 
WARN_ON_ONCE(!in_task()) in access_ok() on x86 (only!), and in_task() just
checks preempt_count.

I guess PeterZ assumed that access_ok() is used only with user space access
APIs (e.g. copy_from_user) which can cause page-fault and locks mm (and might
sleep :)), but now we are trying to use access_ok() with new functions which
disables page-fault and just return -EFAULT.

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>