[PATCH v3] iomap: Fix WARN_ON_ONCE() from unprivileged users

[PATCH v3] iomap: Fix WARN_ON_ONCE() from unprivileged users

[Qian Cai]

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>
---
v3: Keep the default case and update the message.
v2: Record the path, pid and command as well.

 fs/iomap/direct-io.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..b7f3311569bd 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:
@@ -388,6 +389,21 @@ 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.
+		 */
+		path = file_path(dio->iocb->ki_filp, pathname,
+				 sizeof(pathname));
+		if (IS_ERR(path))
+			path = "(unknown)";
+
+		pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %s Comm: %.20s\n",
+				    path, current->comm);
+		return -EIO;
 	default:
 		WARN_ON_ONCE(1);
 		return -EIO;
-- 
2.18.4

Re: [PATCH v3] iomap: Fix WARN_ON_ONCE() from unprivileged users

[Christoph Hellwig]

On Mon, Aug 31, 2020 at 01:25:34PM -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 good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v3] iomap: Fix WARN_ON_ONCE() from unprivileged users

[Matthew Wilcox]

On Mon, Aug 31, 2020 at 01:25:34PM -0400, Qian Cai wrote:
> +	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.
> +		 */
> +		path = file_path(dio->iocb->ki_filp, pathname,
> +				 sizeof(pathname));
> +		if (IS_ERR(path))
> +			path = "(unknown)";
> +
> +		pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %s Comm: %.20s\n",
> +				    path, current->comm);

"File: %pD4"?

Re: [PATCH v3] iomap: Fix WARN_ON_ONCE() from unprivileged users

[Qian Cai]

On Mon, Aug 31, 2020 at 06:42:19PM +0100, Matthew Wilcox wrote:
> On Mon, Aug 31, 2020 at 01:25:34PM -0400, Qian Cai wrote:
> > +	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.
> > +		 */
> > +		path = file_path(dio->iocb->ki_filp, pathname,
> > +				 sizeof(pathname));
> > +		if (IS_ERR(path))
> > +			path = "(unknown)";
> > +
> > +		pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %s Comm: %.20s\n",
> > +				    path, current->comm);
> 
> "File: %pD4"?

Sounds like a good idea. I could use that.