linux-kernel - [PATCH] brd: fix leeping function called from invalid context in brd_insert

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Message-Id: <20250628011459.832760-2-yukuai1@huaweicloud.com>
Date: Sat, 28 Jun 2025 09:14:59 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: axboe@...nel.dk,
	hch@....de
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yukuai3@...wei.com,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: [PATCH] brd: fix leeping function called from invalid context in brd_insert_page()

From: Yu Kuai <yukuai3@...wei.com>

__xa_cmpxchg() is called with rcu_read_lock(), and it will allocated
memory if necessary.

Fix the problem by moving rcu_read_lock() after __xa_cmpxchg, meanwhile,
it still should be held before xa_unlock(), prevent returned page to be
freed by concurrent discard.

Fixes: bbcacab2e8ee ("brd: avoid extra xarray lookups on first write")
Reported-by: syzbot+ea4c8fd177a47338881a@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/685ec4c9.a00a0220.129264.000c.GAE@google.com/
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 drivers/block/brd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index b1be6c510372..0c2eabe14af3 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -64,13 +64,15 @@ static struct page *brd_insert_page(struct brd_device *brd, sector_t sector,
 
 	rcu_read_unlock();
 	page = alloc_page(gfp | __GFP_ZERO | __GFP_HIGHMEM);
-	rcu_read_lock();
-	if (!page)
+	if (!page) {
+		rcu_read_lock();
 		return ERR_PTR(-ENOMEM);
+	}
 
 	xa_lock(&brd->brd_pages);
 	ret = __xa_cmpxchg(&brd->brd_pages, sector >> PAGE_SECTORS_SHIFT, NULL,
 			page, gfp);
+	rcu_read_lock();
 	if (ret) {
 		xa_unlock(&brd->brd_pages);
 		__free_page(page);
-- 
2.39.2