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

On 8/14/21 1:23 AM, Andrew Lunn wrote:

On Fri, Aug 13, 2021 at 07:01:08PM +0300, Pavel Skripkin wrote:

quoted

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@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <redacted>
---
 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