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

[Kees Cook]

On Fri, May 03, 2024 at 01:35:09PM -0600, Jens Axboe wrote:
> On 5/3/24 1:22 PM, Kees Cook wrote:
> > On Fri, May 03, 2024 at 12:49:11PM -0600, Jens Axboe wrote:
> >> On 5/3/24 12:26 PM, Kees Cook wrote:
> >>> 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 */
> >>
> >> Don't think this is sane at all. I'm assuming you meant:
> >>
> >> 	atomic_long_inc_not_zero(&dmabuf->file->f_count);
> > 
> > Oops, yes, sorry. I was typed from memory instead of copy/paste.
> 
> Figured :-)
> 
> >> but won't fly as you're not under RCU in the first place. And what
> >> protects it from being long gone before you attempt this anyway? This is
> >> sane way to attempt to fix it, it's completely opposite of what sane ref
> >> handling should look like.
> >>
> >> Not sure what the best fix is here, seems like dma-buf should hold an
> >> actual reference to the file upfront rather than just stash a pointer
> >> and then later _hope_ that it can just grab a reference. That seems
> >> pretty horrible, and the real source of the issue.
> > 
> > AFAICT, epoll just doesn't hold any references at all. It depends,
> > I think, on eventpoll_release() (really eventpoll_release_file())
> > synchronizing with epoll_wait() (but I don't see how this happens, and
> > the race seems to be against ep_item_poll() ...?)
> >
> > I'm really confused about how eventpoll manages the lifetime of polled
> > fds.
> 
> epoll doesn't hold any references, and it's got some ugly callback to
> deal with that. It's not ideal, nor pretty, but that's how it currently
> works. See eventpoll_release() and how it's called. This means that
> epoll itself is supposedly safe from the file going away, even though it
> doesn't hold a reference to it.

Right -- what remains unclear to me is how struct file lifetime is
expected to work in the struct file_operations::poll callbacks. Because
using get_file() there looks clearly unsafe...

> Except that in this case, the file is already gone by the time
> eventpoll_release() is called. Which presumably is some interaction with
> the somewhat suspicious file reference management that dma-buf is doing.
> But I didn't look into that much, outside of noting it looks a bit
> suspect.

Not yet, though. Here's (one) race state from the analysis. I added lines
for the dma_fence_add_callback()/dma_buf_poll_cb() case, since that's
the case that would escape any eventpoll_release/epoll_wait
synchronization (if it exists?):

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)
                                     dma_fence_add_callback()
eventpoll_release
dmabuf->file deallocation
                                     dma_buf_poll_cb()
                                     fput(dmabuf->file) (f_count == 1)
                                     f_count--
                                     dmabuf->file deallocation

Without fences to create a background callback, we just do a double-free:

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)
                                     dma_buf_poll_cb()
                                     fput(dmabuf->file) (f_count == 1)
                                     f_count--
                                     eventpoll_release
                                     dmabuf->file deallocation
eventpoll_release
dmabuf->file deallocation


get_file(), via epoll_wait()->vfs_poll()->dma_buf_poll(), has raised
f_count again. Then eventpoll_release() is doing things to remove
dmabuf->file from the eventpoll lists, but I *think* this is synchronized
so that an epoll_wait() will only call .poll handlers with a valid
(though possibly f_count==0) file, but I can't figure out where that
happens. (If it's not happening, we have a much bigger problem, but I
imagine we'd see massive corruption all the time, which we don't.)

So, yeah, I can't figure out how eventpoll_release() and epoll_wait()
are expected to behave safely for .poll handlers.

Regardless, for the simple case: it seems like it's just totally illegal
to use get_file() in a poll handler. Is this known/expected? And if so,
how can dmabuf possibly deal with that?

-- 
Kees Cook