Re: [PATCH] PCI: hotplug: set 'thread_finished' flag to be true by default

[Bjorn Helgaas]

[+cc Tong]

On Thu, Jun 10, 2021 at 03:02:41AM +0000, Zheyu Ma wrote:
> In the process of probing the driver 'cpcihp_zt5550', function
> 'pci_get_device' may fail, at this time 'cpci_hp_unregister_controller'
> will be called. Since the default value of 'thread_finished' is 0,
> 'cpci_stop_thread' will be executed, but at this time 'cpci_thread' has
> not been allocated, which leads to a null pointer dereference.

This looks like the same problem Tong reported here:

  https://lore.kernel.org/r/20210321055109.322496-1-ztong0001@xxxxxxxxx

I responded to Tong in that thread, and my response to this patch is
basically the same: I think "thread_finished" is a hack and we can do
better.

> Fix this by set 'thread_finished' to be true by default.
> 
> This log reveals it:

It's good to have the essential details from the log, but most of this
is not relevant to the problem and should be omitted.

> BUG: kernel NULL pointer dereference, address: 0000000000000028
> PGD 0 P4D 0
> Oops: 0002 [#1] PREEMPT SMP PTI
> CPU: 5 PID: 1 Comm: swapper/0 Not tainted 5.12.4-g70e7f0549188-dirty #74
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> RIP: 0010:kthread_stop+0x51/0x380
> Code: b5 4e 10 00 65 8b 05 46 da e1 7e 89 c0 48 0f a3 05 5c c1 ef 05 0f 82 68 01 00 00 e8 99 4e 10 00 4c 8d 6b 28 41 bc 01 00 00 00 <f0> 44 0f c1 63 28 45 85 e4 0f 84 af 02 00 00 e8 7b 4e 10 00 45 85
> RSP: 0000:ffffc90000017ba8 EFLAGS: 00010293
> RAX: ffff888100850000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff811f3b37 RDI: ffffffff86259ca6
> RBP: ffffc90000017bc8 R08: 0000000000000001 R09: 0000000000000001
> R10: 0000000000000001 R11: 00000000ffffffff R12: 0000000000000001
> R13: 0000000000000028 R14: ffff88810160a0c8 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88817b200000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000028 CR3: 0000000006a32000 CR4: 00000000000006e0
> Call Trace:
>  cpci_stop_thread+0x1a/0x30
>  cpci_hp_unregister_controller+0x43/0xc0
>  zt5550_hc_init_one+0x67d/0x720
>  local_pci_probe+0x4f/0xb0
>  pci_device_probe+0x169/0x230
>  ? pci_device_remove+0x110/0x110
>  really_probe+0x283/0x650
>  driver_probe_device+0x89/0x1d0
>  ? mutex_lock_nested+0x1b/0x20
>  device_driver_attach+0x68/0x70
>  __driver_attach+0x11c/0x1b0
>  ? device_driver_attach+0x70/0x70
>  bus_for_each_dev+0xbb/0x110
>  ? rdinit_setup+0x45/0x45
>  driver_attach+0x27/0x30
>  bus_add_driver+0x1eb/0x2a0
>  driver_register+0xa9/0x180
>  __pci_register_driver+0x7c/0x90
>  ? cpci_hotplug_init+0x1c/0x1c
>  zt5550_init+0x66/0x91
>  do_one_initcall+0x7f/0x3d0
>  ? rdinit_setup+0x45/0x45
>  ? rcu_read_lock_sched_held+0x4f/0x80
>  kernel_init_freeable+0x2d7/0x329
>  ? rest_init+0x2c0/0x2c0
>  kernel_init+0x18/0x190
>  ? rest_init+0x2c0/0x2c0
>  ? rest_init+0x2c0/0x2c0
>  ret_from_fork+0x1f/0x30
> Modules linked in:
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> CR2: 0000000000000028
> ---[ end trace ef108f49fd26a20c ]---
> RIP: 0010:kthread_stop+0x51/0x380
> Code: b5 4e 10 00 65 8b 05 46 da e1 7e 89 c0 48 0f a3 05 5c c1 ef 05 0f 82 68 01 00 00 e8 99 4e 10 00 4c 8d 6b 28 41 bc 01 00 00 00 <f0> 44 0f c1 63 28 45 85 e4 0f 84 af 02 00 00 e8 7b 4e 10 00 45 85
> RSP: 0000:ffffc90000017ba8 EFLAGS: 00010293
> RAX: ffff888100850000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff811f3b37 RDI: ffffffff86259ca6
> RBP: ffffc90000017bc8 R08: 0000000000000001 R09: 0000000000000001
> R10: 0000000000000001 R11: 00000000ffffffff R12: 0000000000000001
> R13: 0000000000000028 R14: ffff88810160a0c8 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88817b200000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000028 CR3: 0000000006a32000 CR4: 00000000000006e0
> Kernel panic - not syncing: Fatal exception
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled
> Rebooting in 1 seconds..
> 
> Signed-off-by: Zheyu Ma <zheyuma97@xxxxxxxxx>
> ---
>  drivers/pci/hotplug/cpci_hotplug_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/hotplug/cpci_hotplug_core.c b/drivers/pci/hotplug/cpci_hotplug_core.c
> index d0559d2faf50..d527aae4cc60 100644
> --- a/drivers/pci/hotplug/cpci_hotplug_core.c
> +++ b/drivers/pci/hotplug/cpci_hotplug_core.c
> @@ -47,7 +47,7 @@ static atomic_t extracting;
>  int cpci_debug;
>  static struct cpci_hp_controller *controller;
>  static struct task_struct *cpci_thread;
> -static int thread_finished;
> +static int thread_finished = 1;
>  
>  static int enable_slot(struct hotplug_slot *slot);
>  static int disable_slot(struct hotplug_slot *slot);
> -- 
> 2.17.6
>