Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190215174712.372898450@goodmis.org> <20190215174945.557218316@goodmis.org>
 <CAHk-=wjP4bEYCFaKZVG4kLWmYBctyfsW9jjhC91tiTk9WwB8Rg@mail.gmail.com>
 <20190215171539.4682f0b4@gandalf.local.home> <300C4516-A093-43AE-8707-1C42486807A4@amacapital.net>
 <20190215191949.04604191@gandalf.local.home> <CAHk-=wgJzNp0R3cVhjBPHTR4X9sOvHdqK4UVFfbsOKQ6L=A_eQ@mail.gmail.com>
 <CAHk-=wh9XrOykA5J9RQ7zaBio-S_D+1AE+rGnBYWSd==pCXh+w@mail.gmail.com>
 <20190219111802.1d6dbaa3@gandalf.local.home> <CAHk-=wgTuK3kAduP-gr10vykT1uG=B2VpdffvmyBuTQ1UxPpMg@mail.gmail.com>
 <20190219140330.5dd9e876@gandalf.local.home> <20190220171019.5e81a4946b56982f324f7c45@kernel.org>
In-Reply-To: <20190220171019.5e81a4946b56982f324f7c45@kernel.org>
From:   Jann Horn <jannh@google.com>
Date:   Wed, 20 Feb 2019 14:57:31 +0100
Message-ID: <CAG48ez0e5b0qmW1kMVEYDKvxn=-FkfRw=0KXUm5_R3aAGU-LJA@mail.gmail.com>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        stable <stable@vger.kernel.org>,
        Changbin Du <changbin.du@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Feb 20, 2019 at 9:10 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Tue, 19 Feb 2019 14:03:30 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > > > 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..
> > >
> >
> > Good point. Looks like we should add a new flag for kprobe
> > trace parameters, that tell kprobes if the address is expected to be
> > user or kernel. That would be good regardless of the duplicate
> > meanings, as we could use copy_from_user without touching KERNEL_DS, if
> > the probe argument specifically states "this is user space". For
> > example, when probing do_sys_open, and you want to read what path string
> > was passed into the kernel.
> >
> > Masami, thoughts?
>
> Let me ensure what you want. So you want to access a "string" in user-space,
> not a data structure? In that case, it is very easy to me. It is enough to
> add a "ustring" type to kprobe events. For example, do_sys_opsn's path
> variable is one example. That will be +0(+0(%si)):ustring, and fetcher
> finally copy the string using strncpy_from_user() instead of
> strncpy_from_unsafe(). (*)
[...]
> (*) BTW, there is another concern to use _from_user APIs in kprobe. Are those
> APIs might sleep??

If you want to access userspace without sleeping, and ignore data in
non-present pages, you can do `pagefault_disable(); err =
__copy_from_user_inatomic(...); pagefault_enable();`. (Actually, maybe
the kernel should have a helper for that...)