linux-kernel - [PATCH] media: em28xx: initialize refcount before kref

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Message-Id: <20220122074500.429184-1-dzm91@hust.edu.cn>
Date:   Sat, 22 Jan 2022 15:44:59 +0800
From:   Dongliang Mu <dzm91@...t.edu.cn>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Frank Schäfer <fschaefer.oss@...glemail.com>
Cc:     Dongliang Mu <mudongliangabcd@...il.com>,
        syzkaller <syzkaller@...glegroups.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] media: em28xx: initialize refcount before kref_get

From: Dongliang Mu <mudongliangabcd@...il.com>

The commit 47677e51e2a4("[media] em28xx: Only deallocate struct
em28xx after finishing all extensions") adds kref_get to many init
functions (e.g., em28xx_audio_init). However, kref_init is called too
late in em28xx_usb_probe, since em28xx_init_dev before will invoke
those init functions and call kref_get function. Then refcount bug
occurs in my local syzkaller instance.

Fix it by moving kref_init before em28xx_init_dev. This issue occurs
not only in dev but also dev->dev_next.

Fixes: 47677e51e2a4 ("[media] em28xx: Only deallocate struct em28xx after finishing all extensions")
Reported-by: syzkaller <syzkaller@...glegroups.com>
Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
---
 drivers/media/usb/em28xx/em28xx-cards.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/em28xx/em28xx-cards.c b/drivers/media/usb/em28xx/em28xx-cards.c
index b451ce3cb169..f3b56c065ee1 100644
--- a/drivers/media/usb/em28xx/em28xx-cards.c
+++ b/drivers/media/usb/em28xx/em28xx-cards.c
@@ -3936,6 +3936,8 @@ static int em28xx_usb_probe(struct usb_interface *intf,
 		goto err_free;
 	}
 
+	kref_init(&dev->ref);
+
 	dev->devno = nr;
 	dev->model = id->driver_info;
 	dev->alt   = -1;
@@ -4036,6 +4038,8 @@ static int em28xx_usb_probe(struct usb_interface *intf,
 	}
 
 	if (dev->board.has_dual_ts && em28xx_duplicate_dev(dev) == 0) {
+		kref_init(&dev->dev_next->ref);
+
 		dev->dev_next->ts = SECONDARY_TS;
 		dev->dev_next->alt   = -1;
 		dev->dev_next->is_audio_only = has_vendor_audio &&
@@ -4090,12 +4094,8 @@ static int em28xx_usb_probe(struct usb_interface *intf,
 			em28xx_write_reg(dev, 0x0b, 0x82);
 			mdelay(100);
 		}
-
-		kref_init(&dev->dev_next->ref);
 	}
 
-	kref_init(&dev->ref);
-
 	request_modules(dev);
 
 	/*
-- 
2.25.1