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 11:40:25 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...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>,
        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 Sun, 24 Feb 2019 09:26:45 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, Feb 24, 2019 at 7:18 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Sat, 23 Feb 2019 20:38:03 -0800
> > Andy Lutomirski <luto@...nel.org> wrote:
> > >
> > > 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.
> 
> So the in_task() check does kind of make sense. Using "access_ok()"
> outside of task context is certainly an odd thing, for several
> reasons. The main one being simply that outside of task context, the
> whole "which task" question is open, and you don't know if the task is
> the active one, and so it's not clear if whatever task you interrupt
> might have done "set_fs()" or not.

Ah I got it. Usual case access_ok() in IRQ handler is strange.

> 
> So PeterZ isn't wrong:
> 
> > 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.
> 
> .. but in this case, if we do it all *within* code that saves and
> restores the user access flag with get_fs/set_fs, access_ok() would be
> ok and it doesn't have the above issue.
> 
> So access_ok() in _general_ is absolutely not safe to do from
> interrupts, but within the context of probing user memory from a
> tracing event it just happens to be ok.

Hmm, but user can specify user-memory access from the tracing event
which is located in interrupt handler. So I understand that it is safe
only if we correctly setup access flag with get_fs/set_fs, is that
correct?

> It would be lovely to have a special macro for this, and keep the
> warning for the general case, but because this is a "every
> architecture needs to build their own" it's probably too painful.

Agreed.

> 
> PeterZ, do you remember the particular use case that triggered that
> commit 7c4788950ba5 ("x86/uaccess, sched/preempt: Verify access_ok()
> context")?
> 
>                     Linus


Thank you,

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