linux-kernel - [Linux-kernel-mentees,PATCH] media: hdpvr: Add device num check and handling

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Message-ID: <20190701132206.GA8193@luke-XPS-13>
Date:   Mon, 1 Jul 2019 06:22:06 -0700
From:   Luke Nowakowski-Krijger <lnowakow@....ucsd.edu>
To:     hverkuil@...all.nl
Cc:     mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: [Linux-kernel-mentees,PATCH] media: hdpvr: Add device num check and
 handling

Add hdpvr device num check and error handling

We need to increment the device count atomically before we checkout a
device to make sure that we do not reach the max count, otherwise we get
out-of-bounds errors as reported by syzbot. 

Reported-and-tested-by: syzbot+aac8d0d7205f112045d2@...kaller.appspotmail.com
Signed-off-by: Luke Nowakowski-Krijger <lnowakow@....ucsd.edu>
---
Changes in V1:
+ add storing of incremented index in dev_num var
+ add bounds check on dev_num and appropriate error handling
- remove attomic_inc_return from inside of hdpvr_register call

 drivers/media/usb/hdpvr/hdpvr-core.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/hdpvr/hdpvr-core.c b/drivers/media/usb/hdpvr/hdpvr-core.c
index 29ac7fc5b039..640ef83b57c9 100644
--- a/drivers/media/usb/hdpvr/hdpvr-core.c
+++ b/drivers/media/usb/hdpvr/hdpvr-core.c
@@ -275,6 +275,7 @@ static int hdpvr_probe(struct usb_interface *interface,
 #endif
 	size_t buffer_size;
 	int i;
+	int dev_num;
 	int retval = -ENOMEM;
 
 	/* allocate memory for our device state and initialize it */
@@ -371,9 +372,18 @@ static int hdpvr_probe(struct usb_interface *interface,
 		goto reg_fail;
 	}
 #endif
-
+
+	dev_num = atomic_inc_return(&dev_nr);
+	if (dev_num >= HDPVR_MAX) {
+		v4l2_err(&dev->v4l2_dev,
+			"max device number reached, device register failed\n");
+		atomic_dec(&dev_nr);
+		retval = -ENODEV;
+		goto reg_fail;
+	}
+
 	retval = hdpvr_register_videodev(dev, &interface->dev,
-				    video_nr[atomic_inc_return(&dev_nr)]);
+				    video_nr[dev_num]);
 	if (retval < 0) {
 		v4l2_err(&dev->v4l2_dev, "registering videodev failed\n");
 		goto reg_fail;
-- 
2.20.1