From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752635AbcHNSLL (ORCPT ); Sun, 14 Aug 2016 14:11:11 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:54924 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932421AbcHNR66 (ORCPT ); Sun, 14 Aug 2016 13:58:58 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Peter Zijlstra" , "H. Peter Anvin" , "Steven Rostedt" , "Stephane Eranian" , "Thomas Gleixner" , "Brian Gerst" , "Borislav Petkov" , "Arnaldo Carvalho de Melo" , "Ingo Molnar" , "Masami Hiramatsu" , "Alexander Shishkin" , systemtap@sourceware.org, "Jiri Olsa" , "Vince Weaver" , "Linus Torvalds" , "Andy Lutomirski" , "Denys Vlasenko" , "Ananth N Mavinakayanahalli" Date: Sat, 13 Aug 2016 18:42:51 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.16 187/305] kprobes/x86: Clear TF bit in fault on single-stepping In-Reply-To: X-SA-Exim-Connect-IP: 92.40.249.202 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.37-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Masami Hiramatsu commit dcfc47248d3f7d28df6f531e6426b933de94370d upstream. 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 Reviewed-by: Ananth N Mavinakayanahalli Acked-by: Steven Rostedt Cc: Alexander Shishkin Cc: Andy Lutomirski Cc: Arnaldo Carvalho de Melo Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Cc: systemtap@sourceware.org Link: http://lkml.kernel.org/r/20160611140648.25885.37482.stgit@devbox [ Updated the comments. ] Signed-off-by: Ingo Molnar Signed-off-by: Ben Hutchings --- arch/x86/kernel/kprobes/core.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) --- a/arch/x86/kernel/kprobes/core.c +++ b/arch/x86/kernel/kprobes/core.c @@ -931,7 +931,19 @@ int kprobe_fault_handler(struct pt_regs * 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