Patch "powerpc/jprobes: Disable preemption when triggered through ftrace" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/jprobes: Disable preemption when triggered through ftrace

to the 4.14-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:
powerpc-jprobes-disable-preemption-when-triggered-through-ftrace.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Naveen N. Rao" <[email protected]>
Date: Fri, 22 Sep 2017 14:40:47 +0530
Subject: powerpc/jprobes: Disable preemption when triggered through ftrace

From: Naveen N. Rao <[email protected]>

commit 6baea433bc84cd148af1c524389a8d756f67412e upstream.

KPROBES_SANITY_TEST throws the below splat when CONFIG_PREEMPT is enabled:

Kprobe smoke test: started
DEBUG_LOCKS_WARN_ON(val > preempt_count())

  ------------[ cut here ]------------
  WARNING: CPU: 19 PID: 1 at kernel/sched/core.c:3094 preempt_count_sub+0xcc/0x140
  Modules linked in:
  CPU: 19 PID: 1 Comm: swapper/0 Not tainted 4.13.0-rc7-nnr+ #97
  task: c0000000fea80000 task.stack: c0000000feb00000
  NIP:  c00000000011d3dc LR: c00000000011d3d8 CTR: c000000000a090d0
  REGS: c0000000feb03400 TRAP: 0700   Not tainted  (4.13.0-rc7-nnr+)
  MSR:  8000000000021033 <SF,ME,IR,DR,RI,LE>  CR: 28000282  XER: 00000000
  CFAR: c00000000015aa18 SOFTE: 0
  <snip>
  NIP preempt_count_sub+0xcc/0x140
  LR  preempt_count_sub+0xc8/0x140
  Call Trace:
    preempt_count_sub+0xc8/0x140 (unreliable)
    kprobe_handler+0x228/0x4b0
    program_check_exception+0x58/0x3b0
    program_check_common+0x16c/0x170
    --- interrupt: 0 at kprobe_target+0x8/0x20
                     LR = init_test_probes+0x248/0x7d0
    kp+0x0/0x80 (unreliable)
    livepatch_handler+0x38/0x74
    init_kprobes+0x1d8/0x208
    do_one_initcall+0x68/0x1d0
    kernel_init_freeable+0x298/0x374
    kernel_init+0x24/0x160
    ret_from_kernel_thread+0x5c/0x70
  Instruction dump:
  419effdc 3d22001b 39299240 81290000 2f890000 409effc8 3c82ffcb 3c62ffcb
  3884bc68 3863bc18 4803d5fd 60000000 <0fe00000> 4bffffa8 60000000 60000000
  ---[ end trace 432dd46b4ce3d29f ]---
  Kprobe smoke test: passed successfully

The issue is that we aren't disabling preemption in kprobe_ftrace_handler(). Disable it.

---
 arch/powerpc/kernel/kprobes-ftrace.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

@@ -65,6 +65,7 @@ void kprobe_ftrace_handler(unsigned long
 	/* Disable irq for emulating a breakpoint and avoiding preempt */
 	local_irq_save(flags);
 	hard_irq_disable();
+	preempt_disable();
 
 	p = get_kprobe((kprobe_opcode_t *)nip);
 	if (unlikely(!p) || kprobe_disabled(p))

@@ -86,12 +87,18 @@ void kprobe_ftrace_handler(unsigned long
 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 		if (!p->pre_handler || !p->pre_handler(p, regs))
 			__skip_singlestep(p, regs, kcb, orig_nip);
-		/*
-		 * If pre_handler returns !0, it sets regs->nip and
-		 * resets current kprobe.
-		 */
+		else {
+			/*
+			 * If pre_handler returns !0, it sets regs->nip and
+			 * resets current kprobe. In this case, we still need
+			 * to restore irq, but not preemption.
+			 */
+			local_irq_restore(flags);
+			return;
+		}
 	}
 end:
+	preempt_enable_no_resched();
 	local_irq_restore(flags);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/kprobes-use-synchronize_rcu_tasks-for-optprobe-with-config_preempt-y.patch queue-4.14/powerpc-kprobes-disable-preemption-before-invoking-probe-handler-for-optprobes.patch queue-4.14/powerpc-jprobes-disable-preemption-when-triggered-through-ftrace.patch