ptrace/x86: Partly fix set_task_blockstep()->update_debugctlmsr() logic

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
   needed.

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>

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7a51498..f89cdc6 100644 (file)
--- 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();
 }
 
 /*