linux-kernel - Re: [PATCH 01/12] cachefiles: remove request from xarry during flush requests

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Date: Mon, 6 May 2024 13:50:34 +0800
From: Jingbo Xu <jefflexu@...ux.alibaba.com>
To: Baokun Li <libaokun@...weicloud.com>, netfs@...ts.linux.dev
Cc: dhowells@...hat.com, jlayton@...nel.org, zhujia.zj@...edance.com,
 linux-erofs@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, yangerkun <yangerkun@...wei.com>
Subject: Re: [PATCH 01/12] cachefiles: remove request from xarry during flush
 requests



On 5/6/24 11:57 AM, Baokun Li wrote:
> On 2024/5/6 11:48, Jingbo Xu wrote:
>>
>> On 4/24/24 11:39 AM, libaokun@...weicloud.com wrote:
>>> From: Baokun Li <libaokun1@...wei.com>
>>>
>>> This prevents concurrency from causing access to a freed req.
>> Could you give more details on how the concurrent access will happen?
>> How could another process access the &cache->reqs xarray after it has
>> been flushed?
> 
> Similar logic to restore leading to UAF:
> 
>      mount  |   daemon_thread1    |    daemon_thread2
> ------------------------------------------------------------
>  cachefiles_ondemand_init_object
>   cachefiles_ondemand_send_req
>    REQ_A = kzalloc(sizeof(*req) + data_len)
>    wait_for_completion(&REQ_A->done)
> 
>             cachefiles_daemon_read
>              cachefiles_ondemand_daemon_read
>               REQ_A = cachefiles_ondemand_select_req
>               cachefiles_ondemand_get_fd
>               copy_to_user(_buffer, msg, n)
>             process_open_req(REQ_A)
>                                   // close dev fd
>                                   cachefiles_flush_reqs
>                                    complete(&REQ_A->done)
>    kfree(REQ_A)


>              cachefiles_ondemand_get_fd(REQ_A)
>               fd = get_unused_fd_flags
>               file = anon_inode_getfile
>               fd_install(fd, file)
>               load = (void *)REQ_A->msg.data;
>               load->fd = fd;
>               // load UAF !!!

How could the second cachefiles_ondemand_get_fd() get called here, given
the cache has been flushed and flagged as DEAD?


> 
>>> ---
>>>   fs/cachefiles/daemon.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/fs/cachefiles/daemon.c b/fs/cachefiles/daemon.c
>>> index 6465e2574230..ccb7b707ea4b 100644
>>> --- a/fs/cachefiles/daemon.c
>>> +++ b/fs/cachefiles/daemon.c
>>> @@ -159,6 +159,7 @@ static void cachefiles_flush_reqs(struct
>>> cachefiles_cache *cache)
>>>       xa_for_each(xa, index, req) {
>>>           req->error = -EIO;
>>>           complete(&req->done);
>>> +        __xa_erase(xa, index);
>>>       }
>>>       xa_unlock(xa);
>>>   
> 

-- 
Thanks,
Jingbo