[PATCH] PCI: hv: Fix a use-after-free bug in hv_eject_device_work()

[Michael Kelley]

From: Dexuan Cui <decui at microsoft.com> Sent: Friday, June 21, 2019 12:02 PM
> 
> The commit 05f151a73ec2 itself is correct, but it exposes this
> use-after-free bug, which is caught by some memory debug options.
> 
> Add the Fixes tag to indicate the dependency.
> 
> Fixes: 05f151a73ec2 ("PCI: hv: Fix a memory leak in hv_eject_device_work()")
> Signed-off-by: Dexuan Cui <decui at microsoft.com>
> Cc: stable at vger.kernel.org
> ---
> Sorry for not spotting the bug when sending 05f151a73ec2.
> 
> Now I have enabled the mm debug options to help catch such mistakes in future.
> 
>  drivers/pci/controller/pci-hyperv.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 808a182830e5..42ace1a690f9 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -1880,6 +1880,7 @@ static void hv_pci_devices_present(struct hv_pcibus_device
> *hbus,
>  static void hv_eject_device_work(struct work_struct *work)
>  {
>  	struct pci_eject_response *ejct_pkt;
> +	struct hv_pcibus_device *hbus;
>  	struct hv_pci_dev *hpdev;
>  	struct pci_dev *pdev;
>  	unsigned long flags;
> @@ -1890,6 +1891,7 @@ static void hv_eject_device_work(struct work_struct *work)
>  	} ctxt;
> 
>  	hpdev = container_of(work, struct hv_pci_dev, wrk);
> +	hbus = hpdev->hbus;

In the lines of code following this new assignment, there are four uses of
hpdev->hbus besides the one at the bottom of the function that causes the
use-after-free error.  With 'hbus' now available as a local variable, it looks
rather strange to have those other places still using hpdev->hbus.  I'm thinking
they should be shortened to just 'hbus' for consistency, even though such
changes aren't directly related to fixing the bug.

Michael

> 
>  	WARN_ON(hpdev->state != hv_pcichild_ejecting);
> 
> @@ -1929,7 +1931,9 @@ static void hv_eject_device_work(struct work_struct *work)
>  	/* For the two refs got in new_pcichild_device() */
>  	put_pcichild(hpdev);
>  	put_pcichild(hpdev);
> -	put_hvpcibus(hpdev->hbus);
> +	/* hpdev has been freed. Do not use it any more. */
> +
> +	put_hvpcibus(hbus);
>  }
> 
>  /**
> --
> 2.17.1