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: <20190222235618.dxewmv5dukltaoxl@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 22 Feb 2019 15:56:20 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Miller <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        Andrew Lutomirski <luto@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Fri, Feb 22, 2019 at 03:16:35PM -0800, Linus Torvalds wrote:
> 
> So a kernel pointer value of 0x12345678 could be a value kernel
> pointer pointing to some random kmalloc'ed kernel memory, and a user
> pointer value of 0x12345678 could be a valid _user_ pointer pointing
> to some user mapping.
> 
> See?
> 
> If you access a user pointer, you need to use a user accessor function
> (eg "get_user()"), while if you access a kernel pointer you need to
> just dereference it directly (unless you can't trust it, in which case
> you need to use a _different_ accessor function).

that was clear already.
Reading 0x12345678 via probe_kernel_read can return valid value
and via get_user() can return another valid value on _some_ architectures.

> The fact that user and kernel pointers happen to be distinct on x86-64
> (right now) is just a random implementation detail.

yes and my point that people already rely on this implementation detail.
Say we implement 
int bpf_probe_read(void *val, void *unsafe_ptr)
{
  if (probe_kernel_read(val, unsafe_ptr) == OK) {
     return 0;
  } else (get_user(val, unsafe_ptr) == OK) {
     return 0;
  } else {
     *val = 0;
     return -EFAULT;
  }
}
It will preserve existing bpf_probe_read() behavior on x86.
If x86 implementation changes tomorrow then progs that read user
addresses may start failing randomly because first probe_kernel_read()
will be returning random values from kernel memory and that's no good,
but at least we won't be breaking them today, so we have time to
introduce bpf_user_read and bpf_kernel_read and folks have time to adopt them.

Imo that's much better than making current bpf_probe_read() fail
on user addresses today and not providing a non disruptive path forward.