This is a note to let you know that I've just added the patch titled
ptrace/x86: Partly fix set_task_blockstep()->update_debugctlmsr() logic
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ptrace-x86-partly-fix-set_task_blockstep-update_debugctlmsr-logic.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 667958738244e5be2cb1e05ab1f2a112c52e1c64 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Date: Sat, 11 Aug 2012 18:06:42 +0200
Subject: ptrace/x86: Partly fix set_task_blockstep()->update_debugctlmsr() logic
From: Oleg Nesterov <oleg@xxxxxxxxxx>
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
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@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Rui Xiang <rui.xiang@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/step.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -161,6 +161,16 @@ static void set_task_blockstep(struct ta
{
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 ta
debugctl &= ~DEBUGCTLMSR_BTF;
clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
}
- update_debugctlmsr(debugctl);
+ if (task == current)
+ update_debugctlmsr(debugctl);
+ local_irq_enable();
}
/*
Patches currently in stable-queue which might be from oleg@xxxxxxxxxx are
queue-3.4/list-introduce-list_next_entry-and-list_prev_entry.patch
queue-3.4/ptrace-x86-partly-fix-set_task_blockstep-update_debugctlmsr-logic.patch
queue-3.4/ptrace-x86-introduce-set_task_blockstep-helper.patch
queue-3.4/x86-get_unmapped_area-access-mmap_legacy_base-through-mm_struct-member.patch
--
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