[SRU Bionic] iomap: fix WARN_ON_ONCE() from unprivileged users

[Thadeu Lima de Souza Cascardo]

From: Qian Cai <cai at lca.pw>

BugLink: https://bugs.launchpad.net/bugs/1989144

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 at lca.pw>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong at oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
(backported from commit a805c111650cdba6ee880f528abdd03c1af82089)
[cascardo: code was moved from fs/iomap.c to fs/iomap/direct-io.c]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/iomap.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index b66b047ac1ac..8b022e8fa0a2 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -899,6 +899,16 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 				use_fua = true;
 		}
 		break;
+	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;
-- 
2.34.1