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

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Date: Tue, 7 May 2024 14:52:16 +0800
From: Baokun Li <libaokun@...weicloud.com>
To: Jingbo Xu <jefflexu@...ux.alibaba.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>,
 libaokun@...weicloud.com
Subject: Re: [PATCH 01/12] cachefiles: remove request from xarry during flush
 requests

Hi Jingbo,

Sorry for the late reply.

On 2024/5/6 13:50, Jingbo Xu wrote:
>
> 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?
>
I was in a bit of a rush to reply earlier, and that graph above is
wrong. Please see the one below:

      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
                                   // close dev fd
                                   cachefiles_flush_reqs
                                    complete(&REQ_A->done)
    kfree(REQ_A)
               xa_lock(&cache->reqs);
               cachefiles_ondemand_select_req
                 req->msg.opcode != CACHEFILES_OP_READ
                 // req use-after-free !!!
               xa_unlock(&cache->reqs);
                                    xa_destroy(&cache->reqs)

Even with CACHEFILES_DEAD set, we can still read the requests, so
accessing it after the request has been freed will trigger use-after-free.

Thanks,
Baokun
>>>> ---
>>>>    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);
>>>>