netdev - Re: [syzbot] KMSAN: uninit-value in asix_mdio

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Date:   Sun, 19 Dec 2021 01:01:34 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     syzbot <syzbot+f44badb06036334e867a@...kaller.appspotmail.com>,
        davem@...emloft.net, glider@...gle.com, kuba@...nel.org,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        linux@...pel-privat.de, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KMSAN: uninit-value in asix_mdio_read (2)

On 12/19/21 00:42, Andrew Lunn wrote:
> On Sun, Dec 19, 2021 at 12:14:30AM +0300, Pavel Skripkin wrote:
>> On 12/18/21 14:07, syzbot wrote:
>> > Hello,
>> > 
>> > syzbot found the following issue on:
>> > 
>> > HEAD commit:    b0a8b5053e8b kmsan: core: add dependency on DEBUG_KERNEL
>> > git tree:       https://github.com/google/kmsan.git master
>> > console output: https://syzkaller.appspot.com/x/log.txt?x=13a4d133b00000
>> > kernel config:  https://syzkaller.appspot.com/x/.config?x=46a956fc7a887c60
>> > dashboard link: https://syzkaller.appspot.com/bug?extid=f44badb06036334e867a
>> > compiler:       clang version 14.0.0 (/usr/local/google/src/llvm-git-monorepo 2b554920f11c8b763cd9ed9003f4e19b919b8e1f), GNU ld (GNU Binutils for Debian) 2.35.2
>> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=149fddcbb00000
>> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17baef25b00000
>> > 
>> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> > Reported-by: syzbot+f44badb06036334e867a@...kaller.appspotmail.com
>> > 
>> 
>> The last unhanded case is asix_read_cmd() == 0. Let's handle it...
> 
> That does not look correct, i think.
> 
> asix_read_cmd() == 0 means no error from the read itself. If there is
> no error, we should look at the value of smsr and test for bit

As far as I understand asix_read_cmd() == 0 means 0 bytes was read, 
since it just returns usb_control_msg() return value. I don't know if 
usb_control_msg() can ever return 0, but looks like it's the only 
unhanded case

> AX_HOST_EN. Doing a continue means we just ignore the value returned
> by the good read.
> 
> I think the correct fix is to look at the value of i. If we have
> exceeded 30, we should return -ETIMEDOUT.
> 

Sorry, I don't get how it can help. Report says, that on following line

	else if (smsr & AX_HOST_EN)

smsr is uninitialized. How value of i can help here?

Can you, please, explain if you don't mind :)

Thanks


With regards,
Pavel Skripkin