Patch "powerpc/perf: Fix oops when kthread execs user process" has been added to the 4.11-stable tree

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

powerpc/perf: Fix oops when kthread execs user process

to the 4.11-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-perf-fix-oops-when-kthread-execs-user-process.patch
and it can be found in the queue-4.11 subdirectory.

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

From: Ravi Bangoria <[email protected]>
Date: Thu, 15 Jun 2017 19:16:48 +0530
Subject: powerpc/perf: Fix oops when kthread execs user process

From: Ravi Bangoria <[email protected]>

commit bf05fc25f268cd62f147f368fe65ad3e5b04fe9f upstream.

When a kthread calls call_usermodehelper() the steps are:
1. allocate current->mm
2. load_elf_binary()
3. populate current->thread.regs

While doing this, interrupts are not disabled. If there is a perf interrupt in the middle of this process (i.e. step 1 has completed but not yet reached to step 3) and if perf tries to read userspace regs, kernel oops with following log:

  Unable to handle kernel paging request for data at address 0x00000000
  Faulting instruction address: 0xc0000000000da0fc
  ...
  Call Trace:
  perf_output_sample_regs+0x6c/0xd0
  perf_output_sample+0x4e4/0x830
  perf_event_output_forward+0x64/0x90
  __perf_event_overflow+0x8c/0x1e0
  record_and_restart+0x220/0x5c0
  perf_event_interrupt+0x2d8/0x4d0
  performance_monitor_exception+0x54/0x70
  performance_monitor_common+0x158/0x160
  --- interrupt: f01 at avtab_search_node+0x150/0x1a0
      LR = avtab_search_node+0x100/0x1a0
  ...
  load_elf_binary+0x6e8/0x15a0
  search_binary_handler+0xe8/0x290
  do_execveat_common.isra.14+0x5f4/0x840
  call_usermodehelper_exec_async+0x170/0x210
  ret_from_kernel_thread+0x5c/0x7c

Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace pt_regs are not set.

---
 arch/powerpc/perf/perf_regs.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -101,5 +101,6 @@ void perf_get_regs_user(struct perf_regs
 			struct pt_regs *regs_user_copy)
 {
 	regs_user->regs = task_pt_regs(current);
-	regs_user->abi  = perf_reg_abi(current);
+	regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
+			 PERF_SAMPLE_REGS_ABI_NONE;
 }

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

queue-4.11/powerpc-perf-fix-oops-when-kthread-execs-user-process.patch