linux-ext4 - Re: [PATCH] ext4: fix WARNING in lock_two

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Date: Mon, 25 Dec 2023 02:15:44 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Theodore Ts'o <tytso@....edu>
Cc: Baokun Li <libaokun1@...wei.com>, Edward Adam Davis <eadavis@...com>,
	syzbot+2c4a3b922a860084cc7f@...kaller.appspotmail.com,
	adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com, yangerkun <yangerkun@...wei.com>
Subject: Re: [PATCH] ext4: fix WARNING in lock_two_nondirectories

On Sun, Dec 24, 2023 at 09:11:36PM -0500, Theodore Ts'o wrote:
> On Mon, Dec 25, 2023 at 09:38:51AM +0800, Baokun Li wrote:
> > Marking the boot loader inode as a bad inode here is useless,
> > EXT4_IGET_BAD allows us to get a bad boot loader inode.
> > In my opinion, it doesn't make sense to call lock_two_nondirectories()
> > here to determine if the inode is a regular file or not, since the logic
> > for dealing with non-regular files comes after the locking, so calling
> > lock_two_inodes() directly here will suffice.
> 
> This is all very silly, and why I consider this sort of thing pure
> syzkaller noise.  It really doesn't protect against any real threat,
> and it encourages people to put all sorts of random crud in kernel
> code, all in the name of trying to shut up syzbot.
> 
> If we *are* going to care about shutting up syzkaller, the right
> approach is to simply add a check in swap_inode_boot_loader() which
> causes it to call ext4_error() and declare the file system corrupted
> if the bootloader inode is not a regular file, and then return
> -EFSCORRUPTED.
> 
> We don't need to add random hacks to ext4_iget(), or in other places...

Just check the inode type before anything else and be done with that -
if an in-core inode of a regular file manages to become a directory
right under us, we have a much worse problem.

IOW, the bug is real, but suggested patch is just plain wrong.