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Date:   Sat, 29 Aug 2020 21:30:57 -0400
From:   Qian Cai <cai@....pw>
To:     darrick.wong@...cle.com
Cc:     hch@...radead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Qian Cai <cai@....pw>
Subject: [PATCH] iomap: fix WARN_ON_ONCE() from unprivileged users

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@....pw>
---
 fs/iomap/direct-io.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..6a6b4bc13269 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -389,7 +389,14 @@ 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);
+		/*
+		 * 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("page_mkwrite() is racing with DIO read (iomap->type = %u).\n",
+				    iomap->type);
 		return -EIO;
 	}
 }
-- 
2.18.4

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