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

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Message-ID: <20190228151839.GA159251@google.com>
Date:   Thu, 28 Feb 2019 10:18:39 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Lutomirski <luto@...capital.net>, will.deacon@....com,
        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>,
        Andy Lutomirski <luto@...nel.org>, daniel@...earbox.net,
        netdev@...r.kernel.org, bpf@...r.kernel.org, yhs@...com
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Thu, Feb 28, 2019 at 09:29:13PM +0900, Masami Hiramatsu wrote:
> On Tue, 26 Feb 2019 10:24:47 -0500
> Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> > On Fri, Feb 22, 2019 at 11:27:05AM -0800, Alexei Starovoitov wrote:
> > > On Fri, Feb 22, 2019 at 09:43:14AM -0800, Linus Torvalds wrote:
> > > > 
> > > > Then we should still probably fix up "__probe_kernel_read()" to not
> > > > allow user accesses. The easiest way to do that is actually likely to
> > > > use the "unsafe_get_user()" functions *without* doing a
> > > > uaccess_begin(), which will mean that modern CPU's will simply fault
> > > > on a kernel access to user space.
> > > 
> > > On bpf side the bpf_probe_read() helper just calls probe_kernel_read()
> > > and users pass both user and kernel addresses into it and expect
> > > that the helper will actually try to read from that address.
> > 
> > Slightly related and FWIW, BCC's eBPF-based opensnoop tool [1] installs a
> > kprobe on do_sys_open to monitor calls to the open syscall globally.
> > 
> > do_sys_open() has prototype:
> > 
> > long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode);
> > 
> > This causes a "blank" filename to be displayed by opensnoop when I run it on
> > my Pixel 3 (arm64), possibly because this is a user pointer. However, it
> > works fine on x86-64.
> > 
> > So it seems to me that on arm64, reading user pointers directly still doesn't
> > work even if there is a distinction between user/kernel addresses. In that
> > case reading the user pointer using user accessors (possibly using
> > bpf_probe_user_read helper) should be needed to fix this issue (as Yonghong
> > also privately discussed with me).
> 
> OK, it sounds like the same issue. Please add a bpf_user_read() and use it
> for __user pointer.

CC'd Yonghong who said eariler to me he would add it, but I could add it too
if he wants me to.

thanks,

 - Joel