linux-kernel - [PATCH AUTOSEL 5.4 54/80] iomap: fix WARN_ON

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Date:   Sun, 18 Oct 2020 15:22:05 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Qian Cai <cai@....pw>, Christoph Hellwig <hch@....de>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.4 54/80] iomap: fix WARN_ON_ONCE() from unprivileged users

From: Qian Cai <cai@....pw>

[ Upstream commit a805c111650cdba6ee880f528abdd03c1af82089 ]

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@....pw>
Reviewed-by: Christoph Hellwig <hch@....de>
Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 7b5f76efef02d..8c98fd92bf665 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -377,6 +377,16 @@ 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;
-- 
2.25.1