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: <20190226103558.db7c6b472fccad6e4bca8f56@kernel.org>
Date:   Tue, 26 Feb 2019 10:35:58 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andy Lutomirski <luto@...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 Mon, 25 Feb 2019 11:40:18 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 25 Feb 2019 17:09:45 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > > This should probably go with whatever effort makes nmi_uaccess_ok()
> > > available on all architectures.  That being said, how about just
> > > making copy_from_user_nmi() work on all architectures, even if it just
> > > fails unconditionally on some of them?  
> > 
> > I think even if we have copy_from_user_nmi(), we need something like
> > nmi_uaccess_ok() because without it we can not correctly use
> > __copy_from_user_inatomic()...
> 
> But wouldn't that just be part of the implementation of
> "copy_from_user_nmi()" as being in an NMI just assumes being inatomic?

Yes for copy_from_user_nmi(). But there are some other fundamental
functions, like __get_user(). And when we optimize the loop in
strncpy/strnlen from user in atomic, I think one nmi_access_ok() at
entry is enough.

Thank you,

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