linux-kernel - Re: Bug: soft lockup in exfat_clear

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Message-Id: <79CFA11A-DD34-46B4-8425-74B933ADF447@m.fudan.edu.cn>
Date: Wed, 15 Jan 2025 21:15:40 +0800
From: Kun Hu <huk23@...udan.edu.cn>
To: Namjae Jeon <linkinjeon@...nel.org>
Cc: sj1557.seo@...sung.com,
 yuezhang.mo@...y.com,
 linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 "jjtan24@...udan.edu.cn" <jjtan24@...udan.edu.cn>
Subject: Re: Bug: soft lockup in exfat_clear_bitmap

> This is an already known issue and the relevant patch has been applied.
> Please make sure that the following patch is applied to the kernel you tested.
> 
> a5324b3a488d exfat: fix the infinite loop in __exfat_free_cluster()
> 
> or try to reproduce it with linux-6.13-rc7.

Hi Namjae,

We still successfully reproduced it on the v6.13-rc7. Firstly, I apologize for taking up your time, I’m not sure if this is a significant issue since from the reproducer it kind of looks like it’s caused via fault injection.

The syz_mount_image in the syscall reproducer mounts a randomly generated image and also has the potential to trigger an abnormal path to the file system. Specifically, the . /file0 file is crafted to contain invalid FAT table or bitmap information, it is possible to cause abnormal cyclic behavior in __exfat_free_cluster.

Because p_chain->size is artificially constructed, if it has a large value, then exfat_clear_bitmap will be called frequently. As the call stack shows, the program eventually deadlocks in the loop in __exfat_free_cluster.

This link is a link to our crash log in the rc7 kernel tree:

Link: https://github.com/pghk13/Kernel-Bug/blob/main/0103_6.13rc5_%E6%9C%AA%E6%8A%A5%E5%91%8A/%E6%9C%89%E7%9B%B8%E4%BC%BC%E6%A3%80%E7%B4%A2%E8%AE%B0%E5%BD%95/39-BUG_%20soft%20lockup%20in%20sys_unlink/crashlog0115_rc7.txt

As I said earlier, I'm still consistently reporting the crash I found to you guys now because I'm not sure if this issue is useful to you. If it is not useful, please ignore it. I hope it doesn't take up too much of your time.

———
Kun Hu