From: Jann Horn <jannh@google.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Changbin Du <changbin.du@gmail.com>,
Kees Cook <keescook@chromium.org>,
Andy Lutomirski <luto@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Network Development <netdev@vger.kernel.org>,
bpf@vger.kernel.org, Nadav Amit <namit@vmware.com>,
Rick Edgecombe <rick.p.edgecombe@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault
Date: Fri, 22 Feb 2019 22:43:54 +0100 [thread overview]
Message-ID: <CAG48ez2tVwzhKPu1tvK+E52jUCq5vTwjssrtMaGuCO=kMpSXGw@mail.gmail.com> (raw)
In-Reply-To: <9E670A9A-699C-4B65-962F-CE1AEFD72974@amacapital.net>
(adding some people from the text_poke series to the thread, removing stable@)
On Fri, Feb 22, 2019 at 8:55 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > On Feb 22, 2019, at 11:34 AM, Alexei Starovoitov <alexei.starovoitov@gmail.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@gmail.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.
This lack of logical separation between user and kernel addresses
might interact interestingly with the text_poke series, specifically
"[PATCH v3 05/20] x86/alternative: Initialize temporary mm for
patching" (https://lore.kernel.org/lkml/20190221234451.17632-6-rick.p.edgecombe@intel.com/)
and "[PATCH v3 06/20] x86/alternative: Use temporary mm for text
poking" (https://lore.kernel.org/lkml/20190221234451.17632-7-rick.p.edgecombe@intel.com/),
right? If someone manages to get a tracing BPF program to trigger in a
task that has switched to the patching mm, could they use
bpf_probe_write_user() - which uses probe_kernel_write() after
checking that KERNEL_DS isn't active and that access_ok() passes - to
overwrite kernel text that is mapped writable in the patching mm?
next prev parent reply other threads:[~2019-02-22 21:44 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-15 17:47 [PATCH 0/2 v2] [GIT PULL (take two)] tracing: Two more fixes Steven Rostedt
2019-02-15 17:47 ` [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault Steven Rostedt
2019-02-15 17:55 ` Linus Torvalds
2019-02-15 22:15 ` Steven Rostedt
2019-02-15 23:49 ` Andy Lutomirski
2019-02-16 0:19 ` Steven Rostedt
2019-02-16 1:32 ` Andy Lutomirski
2019-02-16 2:08 ` Steven Rostedt
2019-02-16 2:14 ` Andy Lutomirski
2019-02-16 2:21 ` Steven Rostedt
2019-02-18 17:58 ` Linus Torvalds
2019-02-18 18:23 ` Linus Torvalds
2019-02-19 16:18 ` Steven Rostedt
2019-02-19 18:43 ` Linus Torvalds
2019-02-19 19:03 ` Steven Rostedt
2019-02-20 8:10 ` Masami Hiramatsu
2019-02-20 13:57 ` Jann Horn
2019-02-20 14:47 ` Steven Rostedt
2019-02-20 15:08 ` Masami Hiramatsu
2019-02-20 14:49 ` Steven Rostedt
2019-02-20 16:04 ` Masami Hiramatsu
2019-02-20 16:42 ` Steven Rostedt
2019-02-21 7:37 ` Masami Hiramatsu
2019-02-22 8:27 ` Masami Hiramatsu
2019-02-22 8:35 ` Masami Hiramatsu
2019-02-22 17:43 ` Linus Torvalds
2019-02-22 17:48 ` Andy Lutomirski
2019-02-22 18:28 ` Linus Torvalds
2019-02-22 19:52 ` Andy Lutomirski
2019-02-22 19:27 ` Alexei Starovoitov
2019-02-22 19:30 ` Steven Rostedt
2019-02-22 19:34 ` Alexei Starovoitov
2019-02-22 19:39 ` Steven Rostedt
2019-02-22 19:55 ` Andy Lutomirski
2019-02-22 21:43 ` Jann Horn [this message]
2019-02-22 22:08 ` Nadav Amit
2019-02-22 22:17 ` Jann Horn
2019-02-22 22:21 ` Nadav Amit
2019-02-22 22:39 ` Nadav Amit
2019-02-22 23:02 ` Jann Horn
2019-02-22 23:22 ` Nadav Amit
2019-02-22 23:59 ` Andy Lutomirski
2019-02-23 0:03 ` Alexei Starovoitov
2019-02-23 0:15 ` Nadav Amit
2019-02-24 19:35 ` Andy Lutomirski
2019-02-25 13:36 ` Masami Hiramatsu
2019-02-22 21:20 ` Linus Torvalds
2019-02-22 21:38 ` David Miller
2019-02-22 21:59 ` Linus Torvalds
2019-02-22 22:51 ` Alexei Starovoitov
2019-02-22 23:11 ` Jann Horn
2019-02-22 23:16 ` David Miller
2019-02-22 23:16 ` Linus Torvalds
2019-02-22 23:56 ` Alexei Starovoitov
2019-02-23 0:08 ` Linus Torvalds
2019-02-23 2:28 ` Alexei Starovoitov
2019-02-23 2:32 ` Linus Torvalds
2019-02-23 3:02 ` Steven Rostedt
2019-02-23 4:51 ` Masami Hiramatsu
2019-02-26 3:57 ` Christoph Hellwig
2019-02-26 15:24 ` Joel Fernandes
2019-02-28 12:29 ` Masami Hiramatsu
2019-02-28 15:18 ` Joel Fernandes
2019-02-23 3:47 ` Masami Hiramatsu
2019-02-24 0:44 ` Steven Rostedt
2019-02-24 4:38 ` Andy Lutomirski
2019-02-24 15:17 ` Masami Hiramatsu
2019-02-24 17:26 ` Linus Torvalds
2019-02-25 2:40 ` Masami Hiramatsu
2019-02-25 4:49 ` Andy Lutomirski
2019-02-25 8:09 ` Masami Hiramatsu
2019-02-25 16:40 ` Steven Rostedt
2019-02-26 1:35 ` Masami Hiramatsu
2019-02-25 8:33 ` Peter Zijlstra
2019-02-25 14:52 ` Peter Zijlstra
2019-02-25 16:48 ` Kees Cook
2019-02-25 16:58 ` Andy Lutomirski
2019-02-25 17:07 ` Kees Cook
2019-02-21 7:52 ` Masami Hiramatsu
2019-02-21 14:36 ` Steven Rostedt
2019-02-21 15:58 ` Masami Hiramatsu
2019-02-21 16:16 ` Masami Hiramatsu
2019-02-21 16:32 ` Steven Rostedt
2019-02-23 14:48 ` Masami Hiramatsu
2019-02-15 17:47 ` [PATCH 2/2 v2] tracing: Fix number of entries in trace header Steven Rostedt
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