linux-kernel - [PATCH AUTOSEL 5.10 20/21] xen/blkfront: Only check REQ

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Date:   Wed, 31 May 2023 09:44:13 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Ross Lagerwall <ross.lagerwall@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Sasha Levin <sashal@...nel.org>, sstabellini@...nel.org,
        roger.pau@...rix.com, axboe@...nel.dk,
        xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org
Subject: [PATCH AUTOSEL 5.10 20/21] xen/blkfront: Only check REQ_FUA for writes

From: Ross Lagerwall <ross.lagerwall@...rix.com>

[ Upstream commit b6ebaa8100090092aa602530d7e8316816d0c98d ]

The existing code silently converts read operations with the
REQ_FUA bit set into write-barrier operations. This results in data
loss as the backend scribbles zeroes over the data instead of returning
it.

While the REQ_FUA bit doesn't make sense on a read operation, at least
one well-known out-of-tree kernel module does set it and since it
results in data loss, let's be safe here and only look at REQ_FUA for
writes.

Signed-off-by: Ross Lagerwall <ross.lagerwall@...rix.com>
Acked-by: Juergen Gross <jgross@...e.com>
Link: https://lore.kernel.org/r/20230426164005.2213139-1-ross.lagerwall@citrix.com
Signed-off-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/block/xen-blkfront.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 6f33d62331b1f..d68a8ca2161fb 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -792,7 +792,8 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 		ring_req->u.rw.handle = info->handle;
 		ring_req->operation = rq_data_dir(req) ?
 			BLKIF_OP_WRITE : BLKIF_OP_READ;
-		if (req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA) {
+		if (req_op(req) == REQ_OP_FLUSH ||
+		    (req_op(req) == REQ_OP_WRITE && (req->cmd_flags & REQ_FUA))) {
 			/*
 			 * Ideally we can do an unordered flush-to-disk.
 			 * In case the backend onlysupports barriers, use that.
-- 
2.39.2