Re: [PATCH] net: asix: fix uninit value in asix_mdio_read

[Pavel Skripkin]

On 8/14/21 1:23 AM, Andrew Lunn wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
> > ---
> >  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

Yes, you are right, I missed that, sorry. I will rewrote this loop into 
for loop in v2.

Im wondering why this wrong patch passed KMSAN testing...



With regards,
Pavel Skripkin