ext4: add reserved GDT blocks check

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/0dc2fca8e4f9ac4a40e8424a10163369cca0cc06
• https://git.kernel.org/stable/c/7c921328ac760bba780bdace41f4cd045f7f1405
• https://git.kernel.org/stable/c/b9747263b13e5290ac4d63bec47e38f701303cad
• https://git.kernel.org/stable/c/fba54289176702a7caac0b64738406775817f451
• https://git.kernel.org/stable/c/bfd004a1d3a062aac300523d406ac1f3e5f1a82c
• https://git.kernel.org/stable/c/33b1bba31f4c784d33d2c2517964bdccdc9204cd
• https://git.kernel.org/stable/c/af75c481a2e45e70f62f5942c93695e95bf7bd21
• https://git.kernel.org/stable/c/b55c3cd102a6f48b90e61c44f7f3dda8c290c694