Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

[Jann Horn <jannh@google.com>]

On Fri, Oct 18, 2019 at 4:43 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 10/18/19 8:40 AM, Jann Horn wrote:
> > On Fri, Oct 18, 2019 at 4:37 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >> On 10/18/19 8:34 AM, Jann Horn wrote:
> >>> On Fri, Oct 18, 2019 at 4:01 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>> On 10/17/19 8:41 PM, Jann Horn wrote:
> >>>>> On Fri, Oct 18, 2019 at 4:01 AM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>> This is in preparation for adding opcodes that need to modify files
> >>>>>> in a process file table, either adding new ones or closing old ones.
> >>> [...]
> >>>> Updated patch1:
> >>>>
> >>>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=df6caac708dae8ee9a74c9016e479b02ad78d436
> >>>
> >>> I don't understand what you're doing with old_files in there. In the
> >>> "s->files && !old_files" branch, "current->files = s->files" happens
> >>> without holding task_lock(), but current->files and s->files are also
> >>> the same already at that point anyway. And what's the intent behind
> >>> assigning stuff to old_files inside the loop? Isn't that going to
> >>> cause the workqueue to keep a modified current->files beyond the
> >>> runtime of the work?
> >>
> >> I simply forgot to remove the old block, it should only have this one:
> >>
> >> if (s->files && s->files != cur_files) {
> >>          task_lock(current);
> >>          current->files = s->files;
> >>          task_unlock(current);
> >>          if (cur_files)
> >>                  put_files_struct(cur_files);
> >>          cur_files = s->files;
> >> }
> >
> > Don't you still need a put_files_struct() in the case where "s->files
> > == cur_files"?
>
> I want to hold on to the files for as long as I can, to avoid unnecessary
> shuffling of it. But I take it your worry here is that we'll be calling
> something that manipulates ->files? Nothing should do that, unless
> s->files is set. We didn't hide the workqueue ->files[] before this
> change either.

No, my worry is that the refcount of the files_struct is left too
high. From what I can tell, the "do" loop in io_sq_wq_submit_work()
iterates over multiple instances of struct sqe_submit. If there are
two sqe_submit instances with the same ->files (each holding a
reference from the get_files_struct() in __io_queue_sqe()), then:

When processing the first sqe_submit instance, current->files and
cur_files are set to $user_files.
When processing the second sqe_submit instance, nothing happens
(s->files == cur_files).
After the loop, at the end of the function, put_files_struct() is
called once on $user_files.

So get_files_struct() has been called twice, but put_files_struct()
has only been called once. That leaves the refcount too high, and by
repeating this, an attacker can make the refcount wrap around and then
cause a use-after-free.