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:   Fri, 22 Feb 2019 11:55:31 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.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>,
        Andy Lutomirski <luto@...nel.org>, daniel@...earbox.net,
        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 Feb 22, 2019, at 11:34 AM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
>> On Fri, Feb 22, 2019 at 02:30:26PM -0500, Steven Rostedt wrote:
>> On Fri, 22 Feb 2019 11:27:05 -0800
>> Alexei Starovoitov <alexei.starovoitov@...il.com> 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.
>>> 
>>> If __probe_kernel_read will suddenly start failing on all user addresses
>>> it will break the expectations.
>>> How do we solve it in bpf_probe_read?
>>> Call probe_kernel_read and if that fails call unsafe_get_user byte-by-byte
>>> in the loop?
>>> That's doable, but people already complain that bpf_probe_read() is slow
>>> and shows up in their perf report.
>> 
>> We're changing kprobes to add a specific flag to say that we want to
>> differentiate between kernel or user reads. Can this be done with
>> bpf_probe_read()? If it's showing up in perf report, I doubt a single
> 
> so you're saying you will break existing kprobe scripts?
> I don't think it's a good idea.
> It's not acceptable to break bpf_probe_read uapi.
> 

If so, the uapi is wrong: a long-sized number does not reliably identify an address if you don’t separately know whether it’s a user or kernel address. s390x and 4G:4G x86_32 are the notable exceptions. I have lobbied for RISC-V and future x86_64 to join the crowd.  I don’t know whether I’ll win this fight, but the uapi will probably have to change for at least s390x.

What to do about existing scripts is a different question.