Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove)

[Christian Brauner <brauner@kernel.org>]

On Fri, May 03, 2024 at 11:26:32AM -0700, Kees Cook wrote:
> On Fri, May 03, 2024 at 06:54:22PM +0700, Bui Quang Minh wrote:
> > [...]
> > Root cause:
> > AFAIK, eventpoll (epoll) does not increase the registered file's reference.
> > To ensure the safety, when the registered file is deallocated in __fput,
> > eventpoll_release is called to unregister the file from epoll. When calling
> > poll on epoll, epoll will loop through registered files and call vfs_poll on
> > these files. In the file's poll, file is guaranteed to be alive, however, as
> > epoll does not increase the registered file's reference, the file may be
> > dying
> > and it's not safe the get the file for later use. And dma_buf_poll violates
> > this. In the dma_buf_poll, it tries to get_file to use in the callback. This
> > leads to a race where the dmabuf->file can be fput twice.
> > 
> > Here is the race occurs in the above proof-of-concept
> > 
> > close(dmabuf->file)
> > __fput_sync (f_count == 1, last ref)
> > f_count-- (f_count == 0 now)
> > __fput
> >                                     epoll_wait
> >                                     vfs_poll(dmabuf->file)
> >                                     get_file(dmabuf->file)(f_count == 1)
> > eventpoll_release
> > dmabuf->file deallocation
> >                                     fput(dmabuf->file) (f_count == 1)
> >                                     f_count--
> >                                     dmabuf->file deallocation
> > 
> > I am not familiar with the dma_buf so I don't know the proper fix for the
> > issue. About the rule that don't get the file for later use in poll callback
> > of
> > file, I wonder if it is there when only select/poll exist or just after
> > epoll
> > appears.
> > 
> > I hope the analysis helps us to fix the issue.
> 
> Thanks for doing this analysis! I suspect at least a start of a fix
> would be this:
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 8fe5aa67b167..15e8f74ee0f2 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -267,9 +267,8 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>  
>  		if (events & EPOLLOUT) {
>  			/* Paired with fput in dma_buf_poll_cb */
> -			get_file(dmabuf->file);
> -
> -			if (!dma_buf_poll_add_cb(resv, true, dcb))
> +			if (!atomic_long_inc_not_zero(&dmabuf->file) &&
> +			    !dma_buf_poll_add_cb(resv, true, dcb))
>  				/* No callback queued, wake up any other waiters */
>  				dma_buf_poll_cb(NULL, &dcb->cb);
>  			else
> @@ -290,9 +289,8 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>  
>  		if (events & EPOLLIN) {
>  			/* Paired with fput in dma_buf_poll_cb */
> -			get_file(dmabuf->file);
> -
> -			if (!dma_buf_poll_add_cb(resv, false, dcb))
> +			if (!atomic_long_inc_not_zero(&dmabuf->file) &&
> +			    !dma_buf_poll_add_cb(resv, false, dcb))
>  				/* No callback queued, wake up any other waiters */
>  				dma_buf_poll_cb(NULL, &dcb->cb);
>  			else
> 
> 
> But this ends up leaving "active" non-zero, and at close time it runs
> into:
> 
>         BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
> 
> But the bottom line is that get_file() is unsafe to use in some places,
> one of which appears to be in the poll handler. There are maybe some
> other fragile places too, like in drivers/gpu/drm/vmwgfx/ttm_object.c:
> 
> static bool __must_check get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
> {
> 	return atomic_long_inc_not_zero(&dmabuf->file->f_count) != 0L;
> }
> 
> Which I also note involves a dmabuf...
> 
> Due to this issue I've proposed fixing get_file() to detect pathological states:
> https://lore.kernel.org/lkml/20240502222252.work.690-kees@kernel.org/
> 
> But that has run into some push-back. I'm hoping that seeing this epoll
> example will help illustrate what needs fixing a little better.
> 
> I think the best current proposal is to just WARN sooner instead of a
> full refcount_t implementation:
> 
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8dfd53b52744..e09107d0a3d6 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1040,7 +1040,8 @@ struct file_handle {
>  
>  static inline struct file *get_file(struct file *f)
>  {
> -	atomic_long_inc(&f->f_count);
> +	long prior = atomic_long_fetch_inc_relaxed(&f->f_count);
> +	WARN_ONCE(!prior, "struct file::f_count incremented from zero; use-after-free condition present!\n");
>  	return f;
>  }
>  
> 
> 
> What's the right way to deal with the dmabuf situation? (And I suspect
> it applies to get_dma_buf_unless_doomed() as well...)

No, it doesn't. That's safe afaict as I've explained at lenght in
the other thread.