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

[Baokun Li]

On 2024/5/20 17:10, Jingbo Xu wrote:
> On 5/20/24 4:38 PM, Baokun Li wrote:
> > Hi Jingbo,
> >
> > Thanks for your review!
> >
> > On 2024/5/20 15:24, Jingbo Xu wrote:
> > > 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.
> > > Sorry the above description makes me confused.  As the same request may
> > > be got by different daemon threads multiple times, the introduced
> > > refcount mechanism can't protect it from being completed multiple times
> > > (which is expected).  The refcount only protects it from being freed
> > > multiple times.
> > The idea here is that because the wait only holds one reference count,
> > complete(&req->done) can only be called when the req has been
> > successfully removed from the xarry, otherwise the following UAF may
> > occur:
>
> "complete(&req->done) can only be called when the req has been
> successfully removed from the xarry ..."
>
> How this is done? since the following xarray_erase() following the first
> xarray_erase() will fail as the xarray slot referred by the same id has
> already been erased?

Sorry just forgot to reply to this！

Yes, after loading the xas, the entry (aka req) is checked to see if it 
meets
expectations, and only when it does do we null the xas and complete the 
request.


--
With Best Regards,
Baokun Li