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To:     Qian Cai <cai@lca.pw>, darrick.wong@oracle.com
Cc:     hch@infradead.org, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200831014511.17174-1-cai@lca.pw>
From:   Eric Sandeen <sandeen@sandeen.net>
Subject: Re: [PATCH v2] iomap: Fix WARN_ON_ONCE() from unprivileged users
Message-ID: <d34753a2-57bf-8013-015a-adeb3fe9447c@sandeen.net>
Date:   Mon, 31 Aug 2020 10:48:59 -0500
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On 8/30/20 8:45 PM, 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.
> Thanks Dave Chinner for the initial analysis of the racing reproducers.
> 
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> 
> v2: Record the path, pid and command as well.
> 
>  fs/iomap/direct-io.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index c1aafb2ab990..66a4502ef675 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -374,6 +374,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_dio *dio = data;
> +	char pathname[128], *path;
>  
>  	switch (iomap->type) {
>  	case IOMAP_HOLE:
> @@ -389,7 +390,21 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	case IOMAP_INLINE:
>  		return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
>  	default:
> -		WARN_ON_ONCE(1);

It seems like we should explicitly catch IOMAP_DELALLOC for this case, and leave the
default: as a WARN_ON that is not user-triggerable? i.e.

case IOMAP_DELALLOC:
	<all the fancy warnings>
	return -EIO;
default:
	WARN_ON_ONCE(1);
	return -EIO;

> +		/*
> +		 * 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.
> +		 */
> +		path = file_path(dio->iocb->ki_filp, pathname,
> +				 sizeof(pathname));
> +		if (IS_ERR(path))
> +			path = "(unknown)";
> +
> +		pr_warn_ratelimited("page_mkwrite() is racing with DIO read (iomap->type = %u).\n"
> +				    "File: %s PID: %d Comm: %.20s\n",
> +				    iomap->type, path, current->pid,
> +				    current->comm);

This is very specific ...

Do we know that mmap/page_mkwrite is (and will always be) the only way to reach this
point?

It seems to me that this message won't be very useful for the admin; "pg_mkwrite" may
mean something to us, but doubtful for the general public.  And "type = 1" won't mean
much to the reader, either.

Maybe something like:

"DIO encountered delayed allocation block, racing buffered+direct? File: %s Comm: %.20s\n"

It just seems that a user-facing warning should be something the admin has a chance of
acting on without needing to file a bug for analysis by the developers.

(though TBH "delayed allocation" probably doesn't mean much to the admin, either)

-Eric

>  		return -EIO;
>  	}
>  }
>