Re: [PATCH v2 03/12] cachefiles: fix slab-use-after-free in cachefiles_ondemand_get_fd()

[Jingbo Xu]

On 5/15/24 4:45 PM, libaokun@xxxxxxxxxxxxxxx wrote:
> From: Baokun Li <libaokun1@xxxxxxxxxx>
> 
> We got the following issue in a fuzz test of randomly issuing the restore
> command:
> 
> ==================================================================
> BUG: KASAN: slab-use-after-free in cachefiles_ondemand_daemon_read+0x609/0xab0
> Write of size 4 at addr ffff888109164a80 by task ondemand-04-dae/4962
> 
> CPU: 11 PID: 4962 Comm: ondemand-04-dae Not tainted 6.8.0-rc7-dirty #542
> Call Trace:
>  kasan_report+0x94/0xc0
>  cachefiles_ondemand_daemon_read+0x609/0xab0
>  vfs_read+0x169/0xb50
>  ksys_read+0xf5/0x1e0
> 
> Allocated by task 626:
>  __kmalloc+0x1df/0x4b0
>  cachefiles_ondemand_send_req+0x24d/0x690
>  cachefiles_create_tmpfile+0x249/0xb30
>  cachefiles_create_file+0x6f/0x140
>  cachefiles_look_up_object+0x29c/0xa60
>  cachefiles_lookup_cookie+0x37d/0xca0
>  fscache_cookie_state_machine+0x43c/0x1230
>  [...]
> 
> Freed by task 626:
>  kfree+0xf1/0x2c0
>  cachefiles_ondemand_send_req+0x568/0x690
>  cachefiles_create_tmpfile+0x249/0xb30
>  cachefiles_create_file+0x6f/0x140
>  cachefiles_look_up_object+0x29c/0xa60
>  cachefiles_lookup_cookie+0x37d/0xca0
>  fscache_cookie_state_machine+0x43c/0x1230
>  [...]
> ==================================================================
> 
> Following is the process that triggers the issue:
> 
>      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)
>                                   ------ restore ------
>                                   cachefiles_ondemand_restore
>                                   xas_for_each(&xas, req, ULONG_MAX)
>                                    xas_set_mark(&xas, CACHEFILES_REQ_NEW);
> 
>                                   cachefiles_daemon_read
>                                    cachefiles_ondemand_daemon_read
>                                     REQ_A = cachefiles_ondemand_select_req
> 
>              write(devfd, ("copen %u,%llu", msg->msg_id, size));
>              cachefiles_ondemand_copen
>               xa_erase(&cache->reqs, id)
>               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 !!!
> 
> This issue is caused by issuing a restore command when the daemon is still
> alive, which results in a request being processed multiple times thus
> triggering a UAF. So to avoid this problem, add an additional reference
> count to cachefiles_req, which is held while waiting and reading, and then
> released when the waiting and reading is over.
> 
> Note that since there is only one reference count for waiting, we need to
> avoid the same request being completed multiple times, so we can only
> complete the request if it is successfully removed from the xarray.
> 
> Fixes: e73fa11a356c ("cachefiles: add restore command to recover inflight ondemand read requests")
> Suggested-by: Hou Tao <houtao1@xxxxxxxxxx>
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> Reviewed-by: Jia Zhu <zhujia.zj@xxxxxxxxxxxxx>

How could we protect it from being erased from the xarray with the same
message id in this case?


-- 
Thanks,
Jingbo