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Subject: Re: KASAN: use-after-free in ext4_find_extent in v6.9
From: Shuangpeng Bai <shuangpengbai@gmail.com>
In-Reply-To: <20240515224932.GA202157@mit.edu>
Date: Wed, 15 May 2024 20:33:33 -0400
Cc: linux-ext4@vger.kernel.org,
 linux-kernel@vger.kernel.org,
 syzkaller@googlegroups.com
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References: <5B9F0C1F-C804-4A9C-8597-4E1A7D16B983@gmail.com>
 <20240515224932.GA202157@mit.edu>
To: Theodore Ts'o <tytso@mit.edu>

Hi Ted,

Thanks for your reply!=20

You are right. I disabled CONFIG_BLK_DEV_WRITE_MOUNTED and found this =
bug can not be triggered anymore.=20

I am wondering if there is any suggested way for me to check whether a =
bug is reproduced under a reasonable environment (such as compiling =
config) or not? If so, that would be very helpful.


Best,
Shuangpeng


> On May 15, 2024, at 18:49, Theodore Ts'o <tytso@mit.edu> wrote:
>=20
> On Tue, May 14, 2024 at 08:40:36PM -0400, Shuangpeng Bai wrote:
>> Hi Kernel Maintainers,
>>=20
>> Our tool found a kernel bug KASAN: use-after-free in =
ext4_find_extent. Please see the details below.
>>=20
>> Kernel commit: v6.9 (Commits on May 12, 2024)
>> Kernel config: attachment
>> C/Syz reproducer: attachment
>>=20
>> We find this bug was reported and marked as fixed. =
(https://syzkaller.appspot.com/bug?extid=3D7ec4ebe875a7076ebb31)
>>=20
>> Our reproducer can trigger this bug in v6.9, so the bug may have not =
been fixed correctly.
>=20
> The reason why it was marked as fixed is because the reproducer no
> longer reproduces with CONFIG_BLK_DEV_WRITE_MOUNTED disabled.
> Upstream syzkaller unconditionally disables this config, and we don't
> consider reproducers that have CONFIG_BLK_DEV_WRITE_MOUNTED enabled to
> be a bug.
>=20
> If the reproducer is actively modifying the block device (or the
> underlying file for a loop device) while it is mounted, we don't
> consider this a bug.  This is requires root, and it's no more a
> "security bug" than someone complaining that root can execute a
> reboot(2) system call and calling it a "security bug".
>=20
> I've looked at your "reproducer" and it does appear to be modifying
> the block device while it is mounted, and the config does have
> CONFIG_BLK_DEV_WRITE_MOUNTED enabled.  So I don't care (tm).  If you
> want to put an engineer to work on addressing the bug, and the patch
> is a clean and maintable code fix, I'll certainly consider the change.
> But it's not something that upstream will work on a volunteer basis;
> no company I am aware of is willing to pay for engineers to work on
> this sort of issue.
>=20
> Cheers,
>=20
> - Ted