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

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