Re: [PATCH v4] iomap: fix WARN_ON_ONCE() from unprivileged users

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Aug 31, 2020 at 02:23:53PM -0400, Qian Cai wrote:
> It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
> unprivileged users which would taint the kernel, or worse - panic if
> panic_on_warn or panic_on_taint is set. Hence, just convert it to
> pr_warn_ratelimited() to let users know their workloads are racing.
> Thank Dave Chinner for the initial analysis of the racing reproducers.
> 
> Signed-off-by: Qian Cai <cai@lca.pw>

Looks ok to me too.
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> v4: use %pD4.
> v3: Keep the default case and update the message.
> v2: Record the path, pid and command as well.
> 
>  fs/iomap/direct-io.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index c1aafb2ab990..9519113ebc35 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -388,6 +388,16 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		return iomap_dio_bio_actor(inode, pos, length, dio, iomap);
>  	case IOMAP_INLINE:
>  		return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
> +	case IOMAP_DELALLOC:
> +		/*
> +		 * DIO is not serialised against mmap() access at all, and so
> +		 * if the page_mkwrite occurs between the writeback and the
> +		 * iomap_apply() call in the DIO path, then it will see the
> +		 * DELALLOC block that the page-mkwrite allocated.
> +		 */
> +		pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %pD4 Comm: %.20s\n",
> +				    dio->iocb->ki_filp, current->comm);
> +		return -EIO;
>  	default:
>  		WARN_ON_ONCE(1);
>  		return -EIO;
> -- 
> 2.18.4
>