[PATCH] dma-buf: Fix possible UAF in dma_buf_export

[Christian König]

Am 17.11.22 um 08:48 schrieb Charan Teja Kalla:
> Sometime back Dan also reported the same issue[1] where I do mentioned
> that fput()-->dma_buf_file_release() will remove it from the list.
>
> But it seems that I failed to notice fput() here calls the
> dma_buf_file_release() asynchronously i.e. dmabuf that is accessed in
> the close path is already freed. Am I wrong here?
>
> Should we have the __fput_sync(file) here instead of just fput(file)
> which can solve this problem?
>
> [1]https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F20220516084704.GG29930%40kadam%2F&data=05%7C01%7Cchristian.koenig%40amd.com%7C115292dd7a874278b3ed08dac8701320%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638042680960627756%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=NYNIAJjt%2FSUXoc3wCz2vPvo%2Be%2FIVcABEA2JYZ8%2F2q04%3D&reserved=0

That doesn't look like the right solution to me either.

Essentially we have two separate tear down methods for the dma_buf 
object here:

1. It's not completely initialized and we can call kfree()+module_put() 
to clean up.
     There is actually a dma_resv_fini() here. That should probably be 
fixed.

2. The dma_buf object is fully initialized, but creating the sysfs stats 
file failed.
     In this case we should *not* clean it up like we currently do, but 
rather call fput().

So the right thing to do is a) fix the missing dma_resv_fini() call and 
b) drop the setting d_fsdata=NULL hack and properly return after the fput().

Regards,
Christian.

>
> Thanks,
> Charan
> On 11/17/2022 11:51 AM, Gaosheng Cui wrote:
>> Smatch report warning as follows:
>>
>> drivers/dma-buf/dma-buf.c:681 dma_buf_export() warn:
>>    '&dmabuf->list_node' not removed from list
>>
>> If dma_buf_stats_setup() fails in dma_buf_export(), goto err_sysfs
>> and dmabuf will be freed, but dmabuf->list_node will not be removed
>> from db_list.head, then list traversal may cause UAF.
>>
>> Fix by removeing it from db_list.head before free().
>>
>> Fixes: ef3a6b70507a ("dma-buf: call dma_buf_stats_setup after dmabuf is in valid list")
>> Signed-off-by: Gaosheng Cui <cuigaosheng1 at huawei.com>
>> ---
>>   drivers/dma-buf/dma-buf.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>> index b809513b03fe..6848f50226d5 100644
>> --- a/drivers/dma-buf/dma-buf.c
>> +++ b/drivers/dma-buf/dma-buf.c
>> @@ -675,6 +675,9 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
>>   	return dmabuf;
>>   
>>   err_sysfs:
>> +	mutex_lock(&db_list.lock);
>> +	list_del(&dmabuf->list_node);
>> +	mutex_unlock(&db_list.lock);
>>   	/*
>>   	 * Set file->f_path.dentry->d_fsdata to NULL so that when
>>   	 * dma_buf_release() gets invoked by dentry_ops, it exits