[3.5.y.z,extended,stable] Patch "ptrace/x86: Partly fix" has been added to staging queue

Message ID 1362052750-20717-1-git-send-email-luis.henriques@canonical.com
State New
Headers show

Commit Message

Luis Henriques Feb. 28, 2013, 11:59 a.m.
This is a note to let you know that I have just added a patch titled

    ptrace/x86: Partly fix

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:


If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see



From 04a8df512c72f3373a05b11eaf2bb6d7457545dd Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Sat, 11 Aug 2012 18:06:42 +0200
Subject: [PATCH] ptrace/x86: Partly fix
 set_task_blockstep()->update_debugctlmsr() logic

commit 95cf00fa5d5e2a200a2c044c84bde8389a237e02 upstream.

Afaics the usage of update_debugctlmsr() and TIF_BLOCKSTEP in
step.c was always very wrong.

1. update_debugctlmsr() was simply unneeded. The child sleeps
   TASK_TRACED, __switch_to_xtra(next_p => child) should notice
   TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if

2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register
   should always match the state of current's TIF_BLOCKSTEP bit.

3. Even get_debugctlmsr() + update_debugctlmsr() itself does not
   look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR
   register or the caller can be preempted in between.

4. It is not safe to play with TIF_BLOCKSTEP if task != current.
   DEBUGCTLMSR_BTF and TIF_BLOCKSTEP should always match each
   other if the task is running. The tracee is stopped but it
   can be SIGKILL'ed right before set/clear_tsk_thread_flag().

However, now that uprobes uses user_enable_single_step(current)
we can't simply remove update_debugctlmsr(). So this patch adds
the additional "task == current" check and disables irqs to avoid
the race with interrupts/preemption.

Unfortunately this patch doesn't solve the last problem, we need
another fix. Probably we should teach ptrace_stop() to set/clear
single/block stepping after resume.

And afaics there is yet another problem: perf can play with
MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even
__switch_to_xtra() has problems.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
 arch/x86/kernel/step.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)



diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7a51498..f89cdc6 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -161,6 +161,16 @@  static void set_task_blockstep(struct task_struct *task, bool on)
 	unsigned long debugctl;

+	/*
+	 * Ensure irq/preemption can't change debugctl in between.
+	 * Note also that both TIF_BLOCKSTEP and debugctl should
+	 * be changed atomically wrt preemption.
+	 * FIXME: this means that set/clear TIF_BLOCKSTEP is simply
+	 * wrong if task != current, SIGKILL can wakeup the stopped
+	 * tracee and set/clear can play with the running task, this
+	 * can confuse the next __switch_to_xtra().
+	 */
+	local_irq_disable();
 	debugctl = get_debugctlmsr();
 	if (on) {
 		debugctl |= DEBUGCTLMSR_BTF;
@@ -169,7 +179,9 @@  static void set_task_blockstep(struct task_struct *task, bool on)
 		debugctl &= ~DEBUGCTLMSR_BTF;
 		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
-	update_debugctlmsr(debugctl);
+	if (task == current)
+		update_debugctlmsr(debugctl);
+	local_irq_enable();