ext4: add reserved GDT blocks check

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

ext4: add reserved GDT blocks check

We capture a NULL pointer issue when resizing a corrupt ext4 image which

is freshly clear resize_inode feature (not run e2fsck). It could be

simply reproduced by following steps. The problem is because of the

resize_inode feature was cleared, and it will convert the filesystem to

meta_bg mode in ext4_resize_fs(), but the es->s_reserved_gdt_blocks was

not reduced to zero, so could we mistakenly call reserve_backup_gdb()

and passing an uninitialized resize_inode to it when adding new group

descriptors.

mkfs.ext4 /dev/sda 3G

tune2fs -O ^resize_inode /dev/sda #forget to run requested e2fsck

mount /dev/sda /mnt

resize2fs /dev/sda 8G

========

BUG: kernel NULL pointer dereference, address: 0000000000000028

CPU: 19 PID: 3243 Comm: resize2fs Not tainted 5.18.0-rc7-00001-gfde086c5ebfd #748

...

RIP: 0010:ext4_flex_group_add+0xe08/0x2570

...

Call Trace:

<TASK>

ext4_resize_fs+0xbec/0x1660

__ext4_ioctl+0x1749/0x24e0

ext4_ioctl+0x12/0x20

__x64_sys_ioctl+0xa6/0x110

do_syscall_64+0x3b/0x90

entry_SYSCALL_64_after_hwframe+0x44/0xae

RIP: 0033:0x7f2dd739617b

========

The fix is simple, add a check in ext4_resize_begin() to make sure that

the es->s_reserved_gdt_blocks is zero when the resize_inode feature is

disabled.