linux-kernel - Re: [PATCH] PCI: hv: Fix panic by calling hv_pci_remove

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Message-ID: <20190802194053.GL151852@google.com>
Date:   Fri, 2 Aug 2019 14:40:53 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "driverdev-devel@...uxdriverproject.org" 
        <driverdev-devel@...uxdriverproject.org>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        vkuznets <vkuznets@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        "jackm@...lanox.com" <jackm@...lanox.com>
Subject: Re: [PATCH] PCI: hv: Fix panic by calling hv_pci_remove_slots()
 earlier

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@...rosoft.com>
> Cc: stable@...r.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
>