[PATCH] random: Fix kernel panic due to system_wq use before init

[Waiman Long <Waiman.Long@hpe.com>]

While booting a 4.8-rc6 kernel on a 16-socket 768-thread Broadwell-EX
system, the kernel panic'ed with the following log:

[   51.837010] BUG: unable to handle kernel NULL pointer dereference at 0000000000000102
[   51.845635] IP: [<ffffffff810a49d2>] __queue_work+0x32/0x420
[   52.004366] Call Trace:
[   52.007053]  <IRQ>
[   52.009171]  [<ffffffff810a4de7>] queue_work_on+0x27/0x40
[   52.015306]  [<ffffffff8146ebd7>] credit_entropy_bits+0x1d7/0x2a0
[   52.022002]  [<ffffffff8146f339>] ?  add_interrupt_randomness+0x1b9/0x210
[   52.029366]  [<ffffffff8146f339>] add_interrupt_randomness+0x1b9/0x210
[   52.036544]  [<ffffffff810ea670>] handle_irq_event_percpu+0x40/0x80
[   52.043430]  [<ffffffff810ea6eb>] handle_irq_event+0x3b/0x60
[   52.049655]  [<ffffffff810ede48>] handle_level_irq+0x88/0x100
[   52.055968]  [<ffffffff8103004b>] handle_irq+0xab/0x130
[   52.061708]  [<ffffffff81092861>] ? _local_bh_enable+0x21/0x50
[   52.068125]  [<ffffffff81037bf5>] ? __exit_idle+0x5/0x30
[   52.073965]  [<ffffffff816fc9ed>] do_IRQ+0x4d/0xd0
[   52.079229]  [<ffffffff816fa88c>] common_interrupt+0x8c/0x8c
[   52.085444]  <EOI>
[   52.087568]  [<ffffffff8106bea9>] ? try_to_free_pmd_page+0x9/0x40
[   52.094462]  [<ffffffff8106bea5>] ? try_to_free_pmd_page+0x5/0x40
[   52.101157]  [<ffffffff8106bf2a>] ?  __unmap_pmd_range.part.5+0x4a/0x70
[   52.108330]  [<ffffffff8106c330>] unmap_pmd_range+0x130/0x250
[   52.114644]  [<ffffffff8106cb7b>] __cpa_process_fault+0x47b/0x5a0
[   52.121339]  [<ffffffff8106d7cb>] __change_page_attr+0x78b/0x9e0
[   52.127946]  [<ffffffff81065e45>] ?  __raw_callee_save___native_queued_spin_unlock+0x15/0x30
[   52.137124]  [<ffffffff8106da98>] __change_page_attr_set_clr+0x78/0x300
[   52.144404]  [<ffffffff81229180>] ? __slab_alloc+0x4d/0x5c
[   52.150436]  [<ffffffff8106f15f>] kernel_map_pages_in_pgd+0x8f/0xd0
[   52.157333]  [<ffffffff81dc0fb0>] efi_setup_page_tables+0xcc/0x1d9
[   52.164124]  [<ffffffff81dc0a8d>] efi_enter_virtual_mode+0x35e/0x4af
[   52.171117]  [<ffffffff81d9e109>] start_kernel+0x41f/0x4c8
[   52.177142]  [<ffffffff81d9dad8>] ? set_init_arg+0x55/0x55
[   52.183168]  [<ffffffff81d9d120>] ?  early_idt_handler_array+0x120/0x120
[   52.190440]  [<ffffffff81d9d5d6>] x86_64_start_reservations+0x2a/0x2c
[   52.197516]  [<ffffffff81d9d724>] x86_64_start_kernel+0x14c/0x16f
[   52.204214] Code: 89 e5 41 57 41 56 49 89 f6 41 55 41 89 fd 41 54 49 89 d4 53 48 83 ec 10 89 7d d4 ff 14 25 a0 a7 c2 81 f6 c4 02 0f 85 0d 03 00 00 <41> f6 86 02 01 00 00 01 0f 85 ae 02 00 00 49 c7 c7 18 41 01 00
[   52.225516] RIP  [<ffffffff810a49d2>] __queue_work+0x32/0x420
[   52.231838]  RSP <ffff88bd7f403de8>
[   52.235667] CR2: 0000000000000102
[   52.239667] ---[ end trace 2ee7ea9d2908eb72 ]---
[   52.244743] Kernel panic - not syncing: Fatal exception in interrupt

Looking at the panic'ed instruction indicated that system_wq was
used by credit_entropy_bits() before it it was initialized in an
early_initcall.

This patch prevents the schedule_work() call from being made before
system_wq is initialized.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
 drivers/char/random.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 3efb3bf..3afc519 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -730,8 +730,12 @@ retry:
 		    r->entropy_total >= 2*random_read_wakeup_bits) {
 			struct entropy_store *other = &blocking_pool;
 
-			if (other->entropy_count <=
-			    3 * other->poolinfo->poolfracbits / 4) {
+			/*
+			 * We cannot call schedule_work() before system_wq
+			 * is initialized.
+			 */
+			if (system_wq && (other->entropy_count <=
+			    3 * other->poolinfo->poolfracbits / 4)) {
 				schedule_work(&other->push_work);
 				r->entropy_total = 0;
 			}
-- 
1.7.1