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

[Jens Axboe <axboe@kernel.dk>]

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);

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.

> 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/

I don't think this would catch this case, as the memory could just be
garbage at this point.

-- 
Jens Axboe