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Date:   Fri, 22 Feb 2019 16:03:56 -0800
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Jann Horn <jannh@google.com>, Nadav Amit <namit@vmware.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>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Network Development <netdev@vger.kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Igor Stoppa <igor.stoppa@gmail.com>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault
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On Fri, Feb 22, 2019 at 03:59:30PM -0800, Andy Lutomirski wrote:
> >
> > A relatively simple approach might be to teach BPF not to run kprobe
> > programs and such in contexts where current->mm isn't the active mm?
> > Maybe using nmi_uaccess_okay(), or something like that? It looks like
> > perf_callchain_user() also already uses that. Except that a lot of
> > this code is x86-specific...
> 
> This sounds like exactly the right solution.  If you're running from
> some unknown context (like NMI or tracing), then you should check
> nmi_uaccess_okay().  I think we should just promote that to be a
> non-arch-specific function (that returns true by default) and check it
> the relevant bpf_probe_xyz() functions.
> 
> Alexei, does that seem reasonable?

yep. I think that should work.