linux-kernel - Re: [PATCH AUTOSEL 4.4 11/29] media: m920x: don't use stack on USB reads

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Date:   Thu, 20 Jan 2022 11:26:01 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        rkardell@...a.se, mchehab@...nel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.4 11/29] media: m920x: don't use stack on USB
 reads

Hi!

> From: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
> 
> [ Upstream commit a2ab06d7c4d6bfd0b545a768247a70463e977e27 ]
> 
> Using stack-allocated pointers for USB message data don't work.
> This driver is almost OK with that, except for the I2C read
> logic.
> 
> Fix it by using a temporary read buffer, just like on all other
> calls to m920x_read().

This introduces memory leak... and I don't believe it really fixes the
problem.

> index eafc5c82467f4..5b806779e2106 100644
> --- a/drivers/media/usb/dvb-usb/m920x.c
> +++ b/drivers/media/usb/dvb-usb/m920x.c
> @@ -284,6 +284,13 @@ static int m920x_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[], int nu
>  			/* Should check for ack here, if we knew how. */
>  		}
>  		if (msg[i].flags & I2C_M_RD) {
> +			char *read = kmalloc(1, GFP_KERNEL);
> +			if (!read) {
> +				ret = -ENOMEM;
> +				kfree(read);
> +				goto unlock;
> +			}

kfree(NULL);

>  			for (j = 0; j < msg[i].len; j++) {
>  				/* Last byte of transaction?
>  				 * Send STOP, otherwise send ACK. */
> @@ -291,9 +298,12 @@ static int m920x_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[], int nu
>  
>  				if ((ret = m920x_read(d->udev, M9206_I2C, 0x0,
>  						      0x20 | stop,
> -						      &msg[i].buf[j], 1)) != 0)
> +						      read, 1)) != 0)
>  					goto unlock;

Memory leak of read.

> +				msg[i].buf[j] = read[0];
>  			}
> +
> +			kfree(read);
>  		} else {
>  			for (j = 0; j < msg[i].len; j++) {
>  				/* Last byte of transaction? Then send STOP. */

But more importantly, do we have exact copy of the read problem just
below, during write?

Best regards,
								Pavel

diff --git a/drivers/media/usb/dvb-usb/m920x.c b/drivers/media/usb/dvb-usb/m920x.c
index 691e05833db1..e5ee54324a28 100644
--- a/drivers/media/usb/dvb-usb/m920x.c
+++ b/drivers/media/usb/dvb-usb/m920x.c
@@ -250,7 +250,7 @@ static int m920x_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[], int nu
 {
 	struct dvb_usb_device *d = i2c_get_adapdata(adap);
 	int i, j;
-	int ret = 0;
+	int ret;
 
 	if (mutex_lock_interruptible(&d->i2c_mutex) < 0)
 		return -EAGAIN;
@@ -277,7 +277,6 @@ static int m920x_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[], int nu
 			char *read = kmalloc(1, GFP_KERNEL);
 			if (!read) {
 				ret = -ENOMEM;
-				kfree(read);
 				goto unlock;
 			}
 
@@ -288,8 +287,10 @@ static int m920x_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[], int nu
 
 				if ((ret = m920x_read(d->udev, M9206_I2C, 0x0,
 						      0x20 | stop,
-						      read, 1)) != 0)
+						      read, 1)) != 0) {
+					kfree(read);
 					goto unlock;
+				}
 				msg[i].buf[j] = read[0];
 			}
 


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