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

From: Baokun Li
Date: Tue May 07 2024 - 02:52:35 EST


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@xxxxxxxxxxxxxxx wrote:
From: Baokun Li <libaokun1@xxxxxxxxxx>

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);