Re: [PATCH] PCI: hv: Fix panic by calling hv_pci_remove_slots() earlier

[Bjorn Helgaas <helgaas@kernel.org>]

Hi Dexuan,

The subject line only describes the mechanical code change, which is
obvious from the patch.  It would be better if we could say something
about *why* we need this.

On Fri, Aug 02, 2019 at 01:32:28AM +0000, Dexuan Cui wrote:
> 
> When a slot is removed, the pci_dev must still exist.
> 
> pci_remove_root_bus() removes and free all the pci_devs, so
> hv_pci_remove_slots() must be called before pci_remove_root_bus(),
> otherwise a general protection fault can happen, if the kernel is built

"general protection fault" is an x86 term that doesn't really say what
the issue is.  I suspect this would be a "use-after-free" problem.

> with the memory debugging options.
> 
> Fixes: 15becc2b56c6 ("PCI: hv: Add hv_pci_remove_slots() when we unload the driver")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Cc: stable@vger.kernel.org
> 
> ---
> 
> When pci-hyperv is unloaded, this panic can happen:
> 
>  general protection fault:
>  CPU: 2 PID: 1091 Comm: rmmod Not tainted 5.2.0+
>  RIP: 0010:pci_slot_release+0x30/0xd0
>  Call Trace:
>   kobject_release+0x65/0x190
>   pci_destroy_slot+0x25/0x60
>   hv_pci_remove+0xec/0x110 [pci_hyperv]
>   vmbus_remove+0x20/0x30 [hv_vmbus]
>   device_release_driver_internal+0xd5/0x1b0
>   driver_detach+0x44/0x7c
>   bus_remove_driver+0x75/0xc7
>   vmbus_driver_unregister+0x50/0xbd [hv_vmbus]
>   __x64_sys_delete_module+0x136/0x200
>   do_syscall_64+0x5e/0x220
> 
>  drivers/pci/controller/pci-hyperv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 6b9cc6e60a..68c611d 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -2757,8 +2757,8 @@ static int hv_pci_remove(struct hv_device *hdev)
>  		/* Remove the bus from PCI's point of view. */
>  		pci_lock_rescan_remove();
>  		pci_stop_root_bus(hbus->pci_bus);
> -		pci_remove_root_bus(hbus->pci_bus);
>  		hv_pci_remove_slots(hbus);
> +		pci_remove_root_bus(hbus->pci_bus);

I'm curious about why we need hv_pci_remove_slots() at all.  None of
the other callers of pci_stop_root_bus() and pci_remove_root_bus() do
anything similar to hv_pci_remove_slots().

Surely some of those callers also support slots, so there must be some
other path that calls pci_destroy_slot() in those cases.  Can we use a
similar strategy here?

>  		pci_unlock_rescan_remove();
>  		hbus->state = hv_pcibus_removed;
>  	}
> -- 
> 1.8.3.1
>