Re: [PATCH 2/2] PCI: Fix pci_host_bridge struct device release/free handling

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, May 13, 2020 at 05:38:59PM -0500, Rob Herring wrote:
> The PCI code has several paths where the struct pci_host_bridge is freed
> directly. This is wrong because it contains a struct device which is
> refcounted and should be freed using put_device(). This can result in
> use-after-free errors. I think this problem has existed since 2012 with
> commit 7b5436635800 ("PCI: add generic device into pci_host_bridge
> struct"). It generally hasn't mattered as most host bridge drivers are
> still built-in and can't unbind.
> 
> The problem is a struct device should never be freed directly once
> device_initialize() is called and a ref is held, but that doesn't happen
> until pci_register_host_bridge(). There's then a window between
> allocating the host bridge and pci_register_host_bridge() where kfree
> should be used. This is fragile and requires callers to do the right
> thing. To fix this, we need to split device_register() into
> device_initialize() and device_add() calls, so that the host bridge
> struct is always freed by using a put_device().
> 
> devm_pci_alloc_host_bridge() is using devm_kzalloc() to allocate struct
> pci_host_bridge which will be freed directly. Instead, we can use a
> custom devres action to call put_device().
> 
> Reported-by: Anders Roxell <anders.roxell@linaro.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Signed-off-by: Rob Herring <robh@kernel.org>
> ---
>  drivers/pci/probe.c  | 36 +++++++++++++++++++-----------------
>  drivers/pci/remove.c |  2 +-
>  2 files changed, 20 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index e21dc71b1907..e064ded6fbec 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -565,7 +565,7 @@ static struct pci_bus *pci_alloc_bus(struct pci_bus *parent)
>  	return b;
>  }
>  
> -static void devm_pci_release_host_bridge_dev(struct device *dev)
> +static void pci_release_host_bridge_dev(struct device *dev)
>  {
>  	struct pci_host_bridge *bridge = to_pci_host_bridge(dev);
>  
> @@ -574,12 +574,7 @@ static void devm_pci_release_host_bridge_dev(struct device *dev)
>  
>  	pci_free_resource_list(&bridge->windows);
>  	pci_free_resource_list(&bridge->dma_ranges);
> -}
> -
> -static void pci_release_host_bridge_dev(struct device *dev)
> -{
> -	devm_pci_release_host_bridge_dev(dev);
> -	kfree(to_pci_host_bridge(dev));
> +	kfree(bridge);
>  }
>  
>  static void pci_init_host_bridge(struct pci_host_bridge *bridge)
> @@ -599,6 +594,8 @@ static void pci_init_host_bridge(struct pci_host_bridge *bridge)
>  	bridge->native_pme = 1;
>  	bridge->native_ltr = 1;
>  	bridge->native_dpc = 1;
> +
> +	device_initialize(&bridge->dev);
>  }
>  
>  struct pci_host_bridge *pci_alloc_host_bridge(size_t priv)
> @@ -616,17 +613,25 @@ struct pci_host_bridge *pci_alloc_host_bridge(size_t priv)
>  }
>  EXPORT_SYMBOL(pci_alloc_host_bridge);
>  
> +static void devm_pci_alloc_host_bridge_release(void *data)
> +{
> +	pci_free_host_bridge(data);
> +}
> +
>  struct pci_host_bridge *devm_pci_alloc_host_bridge(struct device *dev,
>  						   size_t priv)
>  {
> +	int ret;
>  	struct pci_host_bridge *bridge;
>  
> -	bridge = devm_kzalloc(dev, sizeof(*bridge) + priv, GFP_KERNEL);
> +	bridge = pci_alloc_host_bridge(priv);
>  	if (!bridge)
>  		return NULL;
>  
> -	pci_init_host_bridge(bridge);
> -	bridge->dev.release = devm_pci_release_host_bridge_dev;
> +	ret = devm_add_action_or_reset(dev, devm_pci_alloc_host_bridge_release,
> +				       bridge);
> +	if (ret)
> +		return NULL;
>  
>  	return bridge;
>  }
> @@ -634,10 +639,7 @@ EXPORT_SYMBOL(devm_pci_alloc_host_bridge);
>  
>  void pci_free_host_bridge(struct pci_host_bridge *bridge)
>  {
> -	pci_free_resource_list(&bridge->windows);
> -	pci_free_resource_list(&bridge->dma_ranges);
> -
> -	kfree(bridge);
> +	put_device(&bridge->dev);
>  }
>  EXPORT_SYMBOL(pci_free_host_bridge);
>  
> @@ -908,7 +910,7 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
>  	if (err)
>  		goto free;
>  
> -	err = device_register(&bridge->dev);
> +	err = device_add(&bridge->dev);
>  	if (err) {
>  		put_device(&bridge->dev);
>  		goto free;
> @@ -978,7 +980,7 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
>  
>  unregister:
>  	put_device(&bridge->dev);
> -	device_unregister(&bridge->dev);
> +	device_del(&bridge->dev);

I think we need to execute device_del() first, then put_device().

Thank you for fixing this code path.

Lorenzo

>  free:
>  	kfree(bus);
> @@ -2953,7 +2955,7 @@ struct pci_bus *pci_create_root_bus(struct device *parent, int bus,
>  	return bridge->bus;
>  
>  err_out:
> -	kfree(bridge);
> +	put_device(&bridge->dev);
>  	return NULL;
>  }
>  EXPORT_SYMBOL_GPL(pci_create_root_bus);
> diff --git a/drivers/pci/remove.c b/drivers/pci/remove.c
> index e9c6b120cf45..95dec03d9f2a 100644
> --- a/drivers/pci/remove.c
> +++ b/drivers/pci/remove.c
> @@ -160,6 +160,6 @@ void pci_remove_root_bus(struct pci_bus *bus)
>  	host_bridge->bus = NULL;
>  
>  	/* remove the host bridge */
> -	device_unregister(&host_bridge->dev);
> +	device_del(&host_bridge->dev);
>  }
>  EXPORT_SYMBOL_GPL(pci_remove_root_bus);
> -- 
> 2.20.1
>