Re: Questions about the patch 054aa8d439b9 ("fget: check that the fd still exists after getting a ref to it")

[libaokun (A)]

在 2022/1/10 17:09, Jann Horn 写道:
> On Wed, Dec 22, 2021 at 11:32 AM libaokun (A) <libaokun1@xxxxxxxxxx> wrote:
> > > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Date: Wed, 1 Dec 2021 10:06:14 -0800
> > > Subject: fget: check that the fd still exists after getting a ref to it
> > >
> > > Jann Horn points out that there is another possible race wrt Unix domain
> > > socket garbage collection, somewhat reminiscent of the one fixed in
> > > commit cbcf01128d0a ("af_unix: fix garbage collect vs MSG_PEEK").
> > >
> > > See the extended comment about the garbage collection requirements added
> > > to unix_peek_fds() by that commit for details.
> > >
> > > The race comes from how we can locklessly look up a file descriptor just
> > > as it is in the process of being closed, and with the right artificial
> > > timing (Jann added a few strategic 'mdelay(500)' calls to do that), the
> > > Unix domain socket garbage collector could see the reference count
> > > decrement of the close() happen before fget() took its reference to the
> > > file and the file was attached onto a new file descriptor.
> > I analyzed this CVE and tried to reproduce it.
> >
> > I guess he triggered it like the stack below.
> >
> >
> > close_fd                               |
> >    pick_file                             |
> >                                          | __fget_files
> > file = files_lookup_fd_rcu(files, fd); |
> >                                          |
> > rcu_assign_pointer(fdt->fd[fd], NULL);
> >    filp_close                            |
> >     fput                                 |
> >                                          | get_file_rcu_many // ned ref>=1
> >      fput_many(file, 1);                 |
> >       file_free(file);                   |
> >                                          |  return file
> >                                          |  // read-after-free
> The race is more complicated than that; you also need to add unix_gc()
> to the race. And if you want to get to memory corruption, you need one
> or two more races involving unix_stream_read_generic() on top of that.
>
> > If you want to successfully execute the get_file_rcu_many function,
> >
> > the reference counting of the file is greater than or equal to 1 and
> >
> > is greater than or equal to 2 after the execution.
> >
> > However, close releases only one reference count and does not release
> > the file,
> >
> > so read-after-free does not occur. So how is the race triggered here?
> This bug does not lead to a UAF of the file, it leads to a locking
> inconsistency between the unix stream read path and the GC.
>
> > The question has been pondered for a long time without any results.
> >
> > Could I get more details (e.g. reproduction methods or stacks) from you ?
> See https://bugs.chromium.org/p/project-zero/issues/detail?id=2247 for
> the original bug report. I'm also working on a more detailed blog
> post, but that isn't finished yet.
> .

Thank you very much for your reply!

With your help, I have understood the problem and successfully 
reproduced it.

Thanks a million!

--
With Best Regards,
Baokun Li