[PATCH 2/2] sched: do not call workqueue sleep hook if task is already dead

From: Roman Pen
Date: Wed Sep 21 2016 - 11:50:03 EST

If panic_on_oops is not set and oops happens inside workqueue kthread,
kernel kills this kthread. Current patch fixes recursive GPF which
happens in that case with the following stack:

[<ffffffff81397f75>] dump_stack+0x68/0x93
[<ffffffff8106954b>] ? do_exit+0x7ab/0xc10
[<ffffffff8108fd73>] __schedule_bug+0x83/0xe0
[<ffffffff81716d5a>] __schedule+0x7ea/0xba0
[<ffffffff810c864f>] ? vprintk_default+0x1f/0x30
[<ffffffff8116a63c>] ? printk+0x48/0x50
[<ffffffff81717150>] schedule+0x40/0x90
[<ffffffff8106976a>] do_exit+0x9ca/0xc10
[<ffffffff810c8e3d>] ? kmsg_dump+0x11d/0x190
[<ffffffff810c8d37>] ? kmsg_dump+0x17/0x190
[<ffffffff81021ee9>] oops_end+0x99/0xd0
[<ffffffff81052da5>] no_context+0x185/0x3e0
[<ffffffff81053083>] __bad_area_nosemaphore+0x83/0x1c0
[<ffffffff810c820e>] ? vprintk_emit+0x25e/0x530
[<ffffffff810531d4>] bad_area_nosemaphore+0x14/0x20
[<ffffffff8105355c>] __do_page_fault+0xac/0x570
[<ffffffff810c66fe>] ? console_trylock+0x1e/0xe0
[<ffffffff81002036>] ? trace_hardirqs_off_thunk+0x1a/0x1c
[<ffffffff81053a2c>] do_page_fault+0xc/0x10
[<ffffffff8171f812>] page_fault+0x22/0x30
[<ffffffff81089bc3>] ? kthread_data+0x33/0x40
[<ffffffff8108427e>] ? wq_worker_sleeping+0xe/0x80
[<ffffffff817169eb>] __schedule+0x47b/0xba0
[<ffffffff81717150>] schedule+0x40/0x90
[<ffffffff8106957d>] do_exit+0x7dd/0xc10
[<ffffffff81021ee9>] oops_end+0x99/0xd0

The root cause is that zeroed task->vfork_done member is accessed from
wq_worker_sleeping() hook. The zeroing out happens on the following


In order to fix a bug dead tasks must be ignored.

Signed-off-by: Roman Pen <roman.penyaev@xxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
kernel/sched/core.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2c303e7..50772e5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3380,8 +3380,22 @@ static void __sched notrace __schedule(bool preempt)
* If a worker went to sleep, notify and ask workqueue
* whether it wants to wake up a task to maintain
* concurrency.
+ *
+ * Also the following stack is possible:
+ * oops_end()
+ * do_exit()
+ * schedule()
+ *
+ * If panic_on_oops is not set and oops happens on
+ * a workqueue execution path, thread will be killed.
+ * That is definitly sad, but not to make the situation
+ * even worse we have to ignore dead tasks in order not
+ * to step on zeroed out members (e.g. t->vfork_done is
+ * already NULL on that path, since we were called by
+ * do_exit()))
- if (prev->flags & PF_WQ_WORKER) {
+ if (prev->flags & PF_WQ_WORKER &&
+ prev->state != TASK_DEAD) {
struct task_struct *to_wakeup;

to_wakeup = wq_worker_sleeping(prev);