Re: [PATCH v2] PCI: Fix Oops caused by uninitialized slot->list in pci_slot_release()

[Bjorn Helgaas]

On Wed, Dec 02, 2020 at 10:33:42AM +0800, Jubin Zhong wrote:
> Once kobject_init_and_add() failed, pci_slot_release() is called to
> delete slot->list from parent->slots. But slot->list is intialized
> afterwards, so we ran into the following crash:
> 
>   Unable to handle kernel NULL pointer dereference at virtual address
> 00000000
>   ...
>   CPU: 10 PID: 1 Comm: swapper/0 Not tainted 4.4.240 #197
>   task: ffffeb398a45ef10 task.stack: ffffeb398a470000
>   PC is at __list_del_entry_valid+0x5c/0xb0
>   LR is at pci_slot_release+0x84/0xe4
>   ...
>   __list_del_entry_valid+0x5c/0xb0
>   pci_slot_release+0x84/0xe4
>   kobject_put+0x184/0x1c4
>   pci_create_slot+0x17c/0x1b4
>   __pci_hp_initialize+0x68/0xa4
>   pciehp_probe+0x1a4/0x2fc
>   pcie_port_probe_service+0x58/0x84
>   driver_probe_device+0x320/0x470
>   __driver_attach+0x54/0xb8
>   bus_for_each_dev+0xc8/0xf0
>   driver_attach+0x30/0x3c
>   bus_add_driver+0x1b0/0x24c
>   driver_register+0x9c/0xe0
>   pcie_port_service_register+0x64/0x7c
>   pcied_init+0x44/0xa4
>   do_one_initcall+0x1d0/0x1f0
>   kernel_init_freeable+0x24c/0x254
>   kernel_init+0x18/0xe8
>   ret_from_fork+0x10/0x20
> 
> Fixes: 8a94644b440e ("PCI: Fix pci_create_slot() reference count leak")
> Signed-off-by: Jubin Zhong <zhongjubin@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx #v4.4.235

I'm curious how you noticed this.  Did kobject_init_and_add() fail for
some reason?  Is there a bug there that we need to fix also?

And why did you mention v4.4.235 in the stable tag?  8a94644b440e
wasn't merged until v5.9.  If 8a94644b440e was backported to older
kernels, we can't know that in general, so I think whoever backported
it is responsible for watching for Fixes tags that mention it.

I updated the stable tag to "v5.9+" and applied this to pci/hotplug
for v5.11, thanks!

> ----
> v2:
>   Since both slot memory and slot->list would be handled by
> pci_slot_release(), we need to make sure slot->list is properly
> initialized beforehand.
> 
> v1: https://lore.kernel.org/lkml/1606288971-47927-1-git-send-email-zhongjubin@xxxxxxxxxx/
>   Two things need to be cleaned up on pci_create_slot's error path:
>   1. free slot memory
>   2. remove slot->list from its parent->slots
>   This patch mistakenly took slot memory as unfreed (which is not), and
> would introduce double free problem.
> ---
>  drivers/pci/slot.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
> index 3861505..ed2077e 100644
> --- a/drivers/pci/slot.c
> +++ b/drivers/pci/slot.c
> @@ -272,6 +272,9 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  		goto err;
>  	}
>  
> +	INIT_LIST_HEAD(&slot->list);
> +	list_add(&slot->list, &parent->slots);
> +
>  	err = kobject_init_and_add(&slot->kobj, &pci_slot_ktype, NULL,
>  				   "%s", slot_name);
>  	if (err) {
> @@ -279,9 +282,6 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  		goto err;
>  	}
>  
> -	INIT_LIST_HEAD(&slot->list);
> -	list_add(&slot->list, &parent->slots);
> -
>  	down_read(&pci_bus_sem);
>  	list_for_each_entry(dev, &parent->devices, bus_list)
>  		if (PCI_SLOT(dev->devfn) == slot_nr)
> -- 
> 1.8.5.6
>