On Sat, Jun 11, 2016 at 11:06:53PM +0900, Masami Hiramatsu wrote: > Fix kprobe_fault_handler to clear TF (trap flag) bit of > flags register in the case of fault fixup on single-stepping. > > If we put a kprobe on the instruction which can cause a > page fault (e.g. actual mov instructions in copy_user_*), > that fault happens on a single-stepping buffer. In this > case, kprobes resets running instance so that the CPU can > retry execution on the original ip address. > However, current code forgets reset TF bit. Since this > fault happens with TF bit set for enabling single-stepping, > when it retries, it causes a debug exception and kprobes > can not handle it because it already reset itself. > > On the most of x86-64 platform, it can be easily reproduced > by using kprobe tracer. E.g. > > # 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. > > Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
Good catch! Reviewed-by: Ananth N Mavinakayanahalli <ana...@linux.vnet.ibm.com>
