linux-kernel - Re: [PATCH] misc: ocxl: fix possible double free in ocxl_file_register

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Date:   Thu, 21 Apr 2022 10:35:52 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>, fbarrat@...ux.ibm.com,
        ajd@...ux.ibm.com, arnd@...db.de, gregkh@...uxfoundation.org,
        alastair@...ilva.org
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] misc: ocxl: fix possible double free in
 ocxl_file_register_afu

On 2022/4/21 06:54, Michael Ellerman wrote:
> Hangyu Hua <hbh25y@...il.com> writes:
>> info_release() will be called in device_unregister() when info->dev's
>> reference count is 0. So there is no need to call ocxl_afu_put() and
>> kfree() again.
> 
> Double frees are often exploitable. But it looks to me like this error
> path is not easily reachable by an attacker.
> 
> ocxl_file_register_afu() is only called from ocxl_probe(), and we only
> go to err_unregister if the sysfs or cdev initialisation fails, which
> should only happen if we hit ENOMEM, or we have a duplicate device which
> would be a device-tree/hardware error. But maybe Fred can check more
> closely, I don't know the driver that well.
> 
> cheers
> 

Hi Michael,

You are right. It is hard to exploit at least in my understanding. 
That's why I didn't give this to security@. But it still need to be fix 
out. By the way, if you are interesting in this kind of bug, you can 
check out the other three patches I submitted recently.

rpmsg: virtio: fix possible double free in rpmsg_virtio_add_ctrl_dev()
https://lore.kernel.org/all/20220418101724.42174-1-hbh25y@gmail.com/

rpmsg: virtio: fix possible double free in rpmsg_probe()
https://lore.kernel.org/all/20220418093144.40859-1-hbh25y@gmail.com/

hwtracing: stm: fix possible double free in stm_register_device()
https://lore.kernel.org/all/20220418081632.35121-1-hbh25y@gmail.com/

They are all the similar bugs i could find in the kernel.

Thanks.

> 
>> Fix this by adding free_minor() and return to err_unregister error path.
>>
>> Fixes: 75ca758adbaf ("ocxl: Create a clear delineation between ocxl backend & frontend")
>> Signed-off-by: Hangyu Hua <hbh25y@...il.com>
>> ---
>>   drivers/misc/ocxl/file.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/misc/ocxl/file.c b/drivers/misc/ocxl/file.c
>> index d881f5e40ad9..6777c419a8da 100644
>> --- a/drivers/misc/ocxl/file.c
>> +++ b/drivers/misc/ocxl/file.c
>> @@ -556,7 +556,9 @@ int ocxl_file_register_afu(struct ocxl_afu *afu)
>>   
>>   err_unregister:
>>   	ocxl_sysfs_unregister_afu(info); // safe to call even if register failed
>> +	free_minor(info);
>>   	device_unregister(&info->dev);
>> +	return rc;
>>   err_put:
>>   	ocxl_afu_put(afu);
>>   	free_minor(info);
>> -- 
>> 2.25.1