linux-kernel - Re: [syzbot] [usb?] KASAN: invalid-free in dev

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Message-ID: <CA+fCnZd2b70N6nXTyWO2UYivh_U7Wey==XWURpFy7B_x8xEFHQ@mail.gmail.com>
Date: Sat, 2 Nov 2024 00:26:30 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Chang Yu <marcus.yu.56@...il.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	syzbot <syzbot+3e563d99e70973c0755c@...kaller.appspotmail.com>
Subject: Re: [syzbot] [usb?] KASAN: invalid-free in dev_free

On Mon, Sep 16, 2024 at 3:24 AM syzbot
<syzbot+3e563d99e70973c0755c@...kaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    68d4209158f4 sub: cdns3: Use predefined PCI vendor ID cons..
> git tree:       https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git usb-testing
> console output: https://syzkaller.appspot.com/x/log.txt?x=10a96200580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cb61872d4d8c5df9
> dashboard link: https://syzkaller.appspot.com/bug?extid=3e563d99e70973c0755c
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1297cc07980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1217c8a9980000

Hi Chang,

I saw your patch testing request for this bug [1] (thank you for
looking into it!), but your patch doesn't appear correct to me, even
though it did satisfy syzbot. One issue that I see with the patch is
calling spin_unlock_irqrestore(&dev->lock) after
kref_put(&dev->count): kref_put() might free the device struct and
spin_unlock_irqrestore() will then do a UAF.

I'm not sure what the correct patch would be though, as I don't
understand what the issue is. It seems that dev_free() indeed gets
called twice, but since it's guarded by kref_put(), this shouldn't
happen AFAIU. Or at least we should get a bad refcount report.

Thanks!

[1] https://lore.kernel.org/all/6721d497.050a0220.35b515.0001.GAE@google.com/T/#mca35ec73d63ff30d7219b6739ed0801688f15f35