md/bitmap: don't set sb values if can't pass sanity check

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

md/bitmap: don't set sb values if can't pass sanity check

If bitmap area contains invalid data, kernel will crash then mdadm

triggers "Segmentation fault".

This is cluster-md speical bug. In non-clustered env, mdadm will

handle broken metadata case. In clustered array, only kernel space

handles bitmap slot info. But even this bug only happened in clustered

env, current sanity check is wrong, the code should be changed.

How to trigger: (faulty injection)

dd if=/dev/zero bs=1M count=1 oflag=direct of=/dev/sda

dd if=/dev/zero bs=1M count=1 oflag=direct of=/dev/sdb

mdadm -C /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sda /dev/sdb

mdadm -Ss

echo aaa > magic.txt

== below modifying slot 2 bitmap data ==

dd if=magic.txt of=/dev/sda seek=16384 bs=1 count=3 <== destroy magic

dd if=/dev/zero of=/dev/sda seek=16436 bs=1 count=4 <== ZERO chunksize

mdadm -A /dev/md0 /dev/sda /dev/sdb

== kernel crashes. mdadm outputs "Segmentation fault" ==

Reason of kernel crash:

In md_bitmap_read_sb (called by md_bitmap_create), bad bitmap magic didn't

block chunksize assignment, and zero value made DIV_ROUND_UP_SECTOR_T()

trigger "divide error".

Crash log:

kernel: md: md0 stopped.

kernel: md/raid1:md0: not clean -- starting background reconstruction

kernel: md/raid1:md0: active with 2 out of 2 mirrors

kernel: dlm: ... ...

kernel: md-cluster: Joined cluster 44810aba-38bb-e6b8-daca-bc97a0b254aa slot 1

kernel: md0: invalid bitmap file superblock: bad magic

kernel: md_bitmap_copy_from_slot can't get bitmap from slot 2

kernel: md-cluster: Could not gather bitmaps from slot 2

kernel: divide error: 0000 [#1] SMP NOPTI

kernel: CPU: 0 PID: 1603 Comm: mdadm Not tainted 5.14.6-1-default

kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)

kernel: RIP: 0010:md_bitmap_create+0x1d1/0x850 [md_mod]

kernel: RSP: 0018:ffffc22ac0843ba0 EFLAGS: 00010246

kernel: ... ...

kernel: Call Trace:

kernel: ? dlm_lock_sync+0xd0/0xd0 [md_cluster 77fe..7a0]

kernel: md_bitmap_copy_from_slot+0x2c/0x290 [md_mod 24ea..d3a]

kernel: load_bitmaps+0xec/0x210 [md_cluster 77fe..7a0]

kernel: md_bitmap_load+0x81/0x1e0 [md_mod 24ea..d3a]

kernel: do_md_run+0x30/0x100 [md_mod 24ea..d3a]

kernel: md_ioctl+0x1290/0x15a0 [md_mod 24ea....d3a]

kernel: ? mddev_unlock+0xaa/0x130 [md_mod 24ea..d3a]

kernel: ? blkdev_ioctl+0xb1/0x2b0

kernel: block_ioctl+0x3b/0x40

kernel: __x64_sys_ioctl+0x7f/0xb0

kernel: do_syscall_64+0x59/0x80

kernel: ? exit_to_user_mode_prepare+0x1ab/0x230

kernel: ? syscall_exit_to_user_mode+0x18/0x40

kernel: ? do_syscall_64+0x69/0x80

kernel: entry_SYSCALL_64_after_hwframe+0x44/0xae

kernel: RIP: 0033:0x7f4a15fa722b

kernel: ... ...

kernel: ---[ end trace 8afa7612f559c868 ]---

kernel: RIP: 0010:md_bitmap_create+0x1d1/0x850 [md_mod]