Skip to content

Commit

Permalink
kprobes/x86: Clear TF bit in fault on single-stepping
Browse files Browse the repository at this point in the history
Fix kprobe_fault_handler() to clear the TF (trap flag) bit of
the flags register in the case of a fault fixup on single-stepping.

If we put a kprobe on the instruction which caused a
page fault (e.g. actual mov instructions in copy_user_*),
that fault happens on the 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 to reset the 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 <mhiramat@kernel.org>
Reviewed-by: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: systemtap@sourceware.org
Cc: stable@vger.kernel.org # All the way back to ancient kernels
Link: http://lkml.kernel.org/r/20160611140648.25885.37482.stgit@devbox
[ Updated the comments. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
  • Loading branch information
Masami Hiramatsu authored and Ingo Molnar committed Jun 14, 2016
1 parent db06d75 commit dcfc472
Showing 1 changed file with 12 additions and 0 deletions.
12 changes: 12 additions & 0 deletions arch/x86/kernel/kprobes/core.c
Original file line number Diff line number Diff line change
Expand Up @@ -961,7 +961,19 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
* normal page fault.
*/
regs->ip = (unsigned long)cur->addr;
/*
* Trap flag (TF) has been set here because this fault
* happened where the single stepping will be done.
* So clear it by resetting the current kprobe:
*/
regs->flags &= ~X86_EFLAGS_TF;

/*
* If the TF flag was set before the kprobe hit,
* don't touch it:
*/
regs->flags |= kcb->kprobe_old_flags;

if (kcb->kprobe_status == KPROBE_REENTER)
restore_previous_kprobe(kcb);
else
Expand Down

0 comments on commit dcfc472

Please sign in to comment.