linux-kernel - [PATCH 3.16 187/305] kprobes/x86: Clear TF bit in fault on single-stepping

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Message-ID: <lsq.1471110171.960206812@decadent.org.uk>
Date:	Sat, 13 Aug 2016 18:42:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Peter Zijlstra" <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Stephane Eranian" <eranian@...gle.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Brian Gerst" <brgerst@...il.com>,
	"Borislav Petkov" <bp@...en8.de>,
	"Arnaldo Carvalho de Melo" <acme@...hat.com>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Masami Hiramatsu" <mhiramat@...nel.org>,
	"Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
	systemtap@...rceware.org, "Jiri Olsa" <jolsa@...hat.com>,
	"Vince Weaver" <vincent.weaver@...ne.edu>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andy Lutomirski" <luto@...nel.org>,
	"Denys Vlasenko" <dvlasenk@...hat.com>,
	"Ananth N Mavinakayanahalli" <ananth@...ux.vnet.ibm.com>
Subject: [PATCH 3.16 187/305] kprobes/x86: Clear TF bit in fault on
 single-stepping

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Masami Hiramatsu <mhiramat@...nel.org>

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 <mhiramat@...nel.org>
Reviewed-by: Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@...dmis.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Cc: systemtap@...rceware.org
Link: http://lkml.kernel.org/r/20160611140648.25885.37482.stgit@devbox
[ Updated the comments. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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