netdev - Re: [PATCH] net: asix: fix uninit value in asix_mdio

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Date:   Sat, 14 Aug 2021 00:23:18 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, linux@...pel-privat.de,
        himadrispandya@...il.com, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+a631ec9e717fb0423053@...kaller.appspotmail.com
Subject: Re: [PATCH] net: asix: fix uninit value in asix_mdio_read

On Fri, Aug 13, 2021 at 07:01:08PM +0300, Pavel Skripkin wrote:
> Syzbot reported uninit-value in asix_mdio_read(). The problem was in
> missing error handling. asix_read_cmd() should initialize passed stack
> variable smsr, but it can fail in some cases. Then while condidition
> checks possibly uninit smsr variable.
> 
> Since smsr is uninitialized stack variable, driver can misbehave,
> because smsr will be random in case of asix_read_cmd() failure.
> Fix it by adding error cheking and just continue the loop instead of
> checking uninit value.
> 
> Fixes: 8a46f665833a ("net: asix: Avoid looping when the device is disconnected")
> Reported-and-tested-by: syzbot+a631ec9e717fb0423053@...kaller.appspotmail.com
> Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
> ---
>  drivers/net/usb/asix_common.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/usb/asix_common.c b/drivers/net/usb/asix_common.c
> index ac92bc52a85e..572ca3077f8f 100644
> --- a/drivers/net/usb/asix_common.c
> +++ b/drivers/net/usb/asix_common.c
> @@ -479,7 +479,13 @@ int asix_mdio_read(struct net_device *netdev, int phy_id, int loc)
>  		usleep_range(1000, 1100);
>  		ret = asix_read_cmd(dev, AX_CMD_STATMNGSTS_REG,
>  				    0, 0, 1, &smsr, 0);
> -	} while (!(smsr & AX_HOST_EN) && (i++ < 30) && (ret != -ENODEV));
> +		if (ret == -ENODEV) {
> +			break;
> +		} else if (ret < 0) {
> +			++i;
> +			continue;
> +		}
> +	} while (!(smsr & AX_HOST_EN) && (i++ < 30));

No ret < 0, don't you end up with a double increment of i? So it will
only retry 15 times, not 30?

Humm.

If ret < 0 is true, smsr is uninitialized? The continue statement
causes a jump into the condition expression, where we evaluate smsr &
AX_HOST_EN. Isn't this just as broken as the original version?

      Andrew