ext4: zero i_disksize when initializing the bootloader inode

In the Linux kernel, the following vulnerability has been resolved:

ext4: zero i_disksize when initializing the bootloader inode

If the boot loader inode has never been used before, the

EXT4_IOC_SWAP_BOOT inode will initialize it, including setting the

i_size to 0. However, if the "never before used" boot loader has a

non-zero i_size, then i_disksize will be non-zero, and the

inconsistency between i_size and i_disksize can trigger a kernel

warning:

WARNING: CPU: 0 PID: 2580 at fs/ext4/file.c:319

CPU: 0 PID: 2580 Comm: bb Not tainted 6.3.0-rc1-00004-g703695902cfa

RIP: 0010:ext4_file_write_iter+0xbc7/0xd10

Call Trace:

vfs_write+0x3b1/0x5c0

ksys_write+0x77/0x160

__x64_sys_write+0x22/0x30

do_syscall_64+0x39/0x80

Reproducer:

1. create corrupted image and mount it:

mke2fs -t ext4 /tmp/foo.img 200

debugfs -wR "sif <5> size 25700" /tmp/foo.img

mount -t ext4 /tmp/foo.img /mnt

cd /mnt

echo 123 > file

2. Run the reproducer program:

posix_memalign(&buf, 1024, 1024)

fd = open("file", O_RDWR | O_DIRECT);

ioctl(fd, EXT4_IOC_SWAP_BOOT);

write(fd, buf, 1024);

Fix this by setting i_disksize as well as i_size to zero when

initiaizing the boot loader inode.