[PATCH] ppc64/perf: Fix oops when kthread execs user process

[Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>]

When a kthread makes a call_usermodehelper() call the steps are:
 a. allocates current->mm
 b. load_elf_binary()
 c. populates 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 'a' has completed
but not yet reached to step 'c') and if perf tries to read userspace
regs, kernel oops with following log:

  [  131.217172] Unable to handle kernel paging request for data at address 0x00000000
  [  131.217731] Faulting instruction address: 0xc0000000000da0fc
  ...
  [  131.235555] Call Trace:
  [  131.235714] [c0000000bbaaad60] [c00000000025dedc] perf_output_sample_regs+0x6c/0xd0
  [  131.236020] [c0000000bbaaadb0] [c000000000269b44] perf_output_sample+0x4e4/0x830
  [  131.236362] [c0000000bbaaae40] [c00000000026a354] perf_event_output_forward+0x64/0x90
  [  131.236668] [c0000000bbaaaeb0] [c00000000026298c] __perf_event_overflow+0x8c/0x1e0
  [  131.236979] [c0000000bbaaaf00] [c0000000000dc330] record_and_restart+0x220/0x5c0
  [  131.237306] [c0000000bbaab230] [c0000000000dd1d8] perf_event_interrupt+0x2d8/0x4d0
  [  131.237611] [c0000000bbaab320] [c0000000000294a4] performance_monitor_exception+0x54/0x70
  [  131.237891] [c0000000bbaab350] [c00000000000a0a8] performance_monitor_common+0x158/0x160
  [  131.238208] --- interrupt: f01 at avtab_search_node+0x150/0x1a0
  [  131.238208]     LR = avtab_search_node+0x100/0x1a0
  [  131.238617] [c0000000bbaab640] [c000000000526770] context_struct_compute_av+0x220/0x5b0 (unreliable)
  [  131.238948] [c0000000bbaab730] [c0000000005278b4] security_compute_av+0x174/0x390
  [  131.239231] [c0000000bbaab7e0] [c0000000005050e4] avc_compute_av+0x84/0x260
  [  131.239471] [c0000000bbaab890] [c000000000506198] avc_has_perm+0xf8/0x1c0
  [  131.239708] [c0000000bbaab980] [c00000000050f32c] file_has_perm+0x6c/0xd0
  [  131.239972] [c0000000bbaab9e0] [c0000000004ff0fc] security_mmap_file+0xac/0x140
  [  131.240256] [c0000000bbaaba50] [c0000000002b1fc0] vm_mmap_pgoff+0x80/0x160
  [  131.240532] [c0000000bbaabb30] [c0000000003f7db4] elf_map+0xa4/0x180
  [  131.240771] [c0000000bbaabb90] [c0000000003f9a48] load_elf_binary+0x6e8/0x15a0
  [  131.241060] [c0000000bbaabc90] [c000000000374f58] search_binary_handler+0xe8/0x290
  [  131.241347] [c0000000bbaabd20] [c000000000375c14] do_execveat_common.isra.14+0x5f4/0x840
  [  131.241631] [c0000000bbaabdf0] [c00000000010be70] call_usermodehelper_exec_async+0x170/0x210
  [  131.241955] [c0000000bbaabe30] [c00000000000bae0] ret_from_kernel_thread+0x5c/0x7c

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

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
---
Note: this should go to stable as well. I've not checked below 4.4
kernel but I'm able to reproduce it with 4.4 kernel.

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

diff --git a/arch/powerpc/perf/perf_regs.c b/arch/powerpc/perf/perf_regs.c
index cbd82fd..09ceea6 100644
--- a/arch/powerpc/perf/perf_regs.c
+++ b/arch/powerpc/perf/perf_regs.c
@@ -101,5 +101,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
 			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;
 }
-- 
2.9.4