Re: [PATCH] ext4: fix WARNING in lock_two_nondirectories

[Al Viro <viro@zeniv.linux.org.uk>]

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.