linux-ext4 - Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find

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Date:   Mon, 2 Jan 2023 07:41:18 +0200
From:   Tudor Ambarus <tudor.ambarus@...aro.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        joneslee@...gle.com,
        syzbot+0827b4b52b5ebf65f219@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find_extent



On 30.12.2022 22:07, Eric Biggers wrote:
> On Fri, Dec 30, 2022 at 01:42:45PM +0200, Tudor Ambarus wrote:
>>
>> Seems that __ext4_iget() is not called on writes.
> 
> It is called when the inode is first accessed.  Usually that's when the file is
> opened.

Okay, thanks.

> 
> So the question is why didn't it validate the inode's extent header, or
> alternatively how did the inode's extent header get corrupted afterwards.
> 
>> You can find below the sequence of calls that leads to the bug.
> 
> A stack trace is not a reproducer.  Things must have happened before that point.
> 

I will try to dig more to understand what's happening. If you like to
take a look into the reproducer, here it is:
https://syzkaller.appspot.com/text?tag=ReproC&x=17beb560480000

The reproducer was used for Android 5.15 and the bug is reported at [1],
but as I mentioned earlier, using the same reproducer and config I hit
the bug on v6.2-rc1 as well.

Thanks for the help.
ta

[1] 
https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd