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Message-ID: <c1add351-f132-60d5-7ed8-cba1061d98b1@quicinc.com>
Date:   Fri, 27 Oct 2023 11:53:35 +0530
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Subject: Re: [PATCH] Devcoredump: fix use-after-free issue when releasing
 devcd device
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To:     Yu Wang <quic_yyuwang@quicinc.com>, <johannes@sipsolutions.net>,
        <gregkh@linuxfoundation.org>, <rafael@kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <kernel@quicinc.com>
References: <20231027055521.2679-1-quic_yyuwang@quicinc.com>
From:   Mukesh Ojha <quic_mojha@quicinc.com>
In-Reply-To: <20231027055521.2679-1-quic_yyuwang@quicinc.com>
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On 10/27/2023 11:25 AM, Yu Wang wrote:
> With sample code as below, it may hit use-after-free issue when
> releasing devcd device.
> 
>      struct my_coredump_state {
>          struct completion dump_done;
>          ...
>      };
> 
>      static void my_coredump_free(void *data)
>      {
>          struct my_coredump_state *dump_state = data;
>          ...
>          complete(&dump_state->dump_done);
>      }
> 
>      static void my_dev_release(struct device *dev)
>      {
>          kfree(dev);
>      }
> 
>      static void my_coredump()
>      {
>          struct my_coredump_state dump_state;
>          struct device *new_device =
>              kzalloc(sizeof(*new_device), GFP_KERNEL);
> 
>          ...
>          new_device->release = my_dev_release;
>          device_initialize(new_device);
>          ...
>          device_add(new_device);
>          ...
>          init_completion(&dump_state.dump_done);
>          dev_coredumpm(new_device, NULL, &dump_state, datalen, GFP_KERNEL,
>                        my_coredump_read, my_coredump_free);
>          wait_for_completion(&dump_state.dump_done);
>          device_del(new_device);
>          put_device(new_device);
>      }
> 
> In devcoredump framework, devcd_dev_release() will be called when
> releasing the devcd device, it will call the free() callback first
> and try to delete the symlink in sysfs directory of the failing device.
> Eventhough it has checked 'devcd->failing_dev->kobj.sd' before that,
> there is no mechanism to ensure it's still available when accessing
> it in kernfs_find_ns(), refer to the diagram as below:
> 
>      Thread A was waiting for 'dump_state.dump_done' at #A-1-2 after
>      calling dev_coredumpm().
>      When thread B calling devcd->free() at #B-2-1, it wakes up
>      thread A from point #A-1-2, which will call device_del() to
>      delete the device.
>      If #B-2-2 comes before #A-3-1, but #B-4 comes after #A-4, it
>      will hit use-after-free issue when trying to access
>      'devcd->failing_dev->kobj.sd'.
> 
>      #A-1-1: dev_coredumpm()
>        #A-1-2: wait_for_completion(&dump_state.dump_done)
>        #A-1-3: device_del()
>          #A-2: kobject_del()
>            #A-3-1: sysfs_remove_dir() --> set kobj->sd=NULL
>            #A-3-2: kernfs_put()
>              #A-4: kmem_cache_free() --> free kobj->sd
> 
>      #B-1: devcd_dev_release()
>        #B-2-1: devcd->free(devcd->data)
>        #B-2-2: check devcd->failing_dev->kobj.sd
>        #B-2-3: sysfs_delete_link()
>          #B-3: kernfs_remove_by_name_ns()
>            #B-4: kernfs_find_ns() --> access devcd->failing_dev->kobj.sd
> 
> To fix this issue, put operations on devcd->failing_dev before
> calling the free() callback in devcd_dev_release().
> 
> Signed-off-by: Yu Wang <quic_yyuwang@quicinc.com>
> ---
>   drivers/base/devcoredump.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
> index 91536ee05f14..35c704ddfeae 100644
> --- a/drivers/base/devcoredump.c
> +++ b/drivers/base/devcoredump.c
> @@ -83,9 +83,6 @@ static void devcd_dev_release(struct device *dev)
>   {
>   	struct devcd_entry *devcd = dev_to_devcd(dev);
>   
> -	devcd->free(devcd->data);
> -	module_put(devcd->owner);
> -
>   	/*
>   	 * this seems racy, but I don't see a notifier or such on
>   	 * a struct device to know when it goes away?

Does this comment became obsolete now ?

-Mukesh

> @@ -95,6 +92,8 @@ static void devcd_dev_release(struct device *dev)
>   				  "devcoredump");
>   
>   	put_device(devcd->failing_dev);
> +	devcd->free(devcd->data);
> +	module_put(devcd->owner);
>   	kfree(devcd);
>   }
>