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    SubjectRe: [PATCH tip/master] [BUGFIX] kprobes/x86: Fix to clear TF bit in fault-on-single-stepping

    * Steven Rostedt <rostedt@goodmis.org> wrote:

    > On Mon, 13 Jun 2016 19:13:45 -0400
    > Steven Rostedt <rostedt@goodmis.org> wrote:
    >
    >
    > > > # cd /sys/kernel/debug/tracing
    > > > # echo p copy_user_enhanced_fast_string+5 > kprobe_events
    > > > # echo 1 > events/kprobes/enable
    > > >
    > > > And you'll see a kernel panic on do_debug(), since the debug
    > > > trap is not handled by kprobes.
    > > >
    > > > To fix this problem, we just need to clear the TF bit when
    > > > resetting running kprobe.
    > > >
    > >
    > > This should definitely be marked for stable, and I bisected it all the
    > > way down to this commit: f4cb1cc18f364d "x86-64, copy_user: Remove zero
    > > byte check before copy user buffer."
    > >
    > > I reverted that commit and sure enough, this bug goes away. I'm not
    > > saying the revert should be done. I'm just doing an FYI, and showing how
    > > changes that appear to be a nice clean up can have subtle effects. I'm
    > > not even sure how that change caused this to be a problem with kprobes.
    > >
    >
    > Nevermind, reverting that commit only moved the location of the
    > "rep movsb" that you were placing the kprobe on. When I do:
    >
    > echo "p copy_user_enhanced_fast_string+9" > kprobe_events
    >
    > I get the same result.
    >
    > That means we need to make that stable tag even earlier.

    I added:

    Cc: stable@vger.kernel.org # All the way back to ancient kernels

    Thanks,

    Ingo

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