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

[Baokun Li]

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);
> > > >