btrfs: fix a NULL pointer dereference when failed to start a new trasacntion

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

btrfs: fix a NULL pointer dereference when failed to start a new trasacntion

[BUG]

Syzbot reported a NULL pointer dereference with the following crash:

FAULT_INJECTION: forcing a failure.

start_transaction+0x830/0x1670 fs/btrfs/transaction.c:676

prepare_to_relocate+0x31f/0x4c0 fs/btrfs/relocation.c:3642

relocate_block_group+0x169/0xd20 fs/btrfs/relocation.c:3678

...

BTRFS info (device loop0): balance: ended with status: -12

Oops: general protection fault, probably for non-canonical address 0xdffffc00000000cc: 0000 [#1] PREEMPT SMP KASAN NOPTI

KASAN: null-ptr-deref in range [0x0000000000000660-0x0000000000000667]

RIP: 0010:btrfs_update_reloc_root+0x362/0xa80 fs/btrfs/relocation.c:926

Call Trace:

<TASK>

commit_fs_roots+0x2ee/0x720 fs/btrfs/transaction.c:1496

btrfs_commit_transaction+0xfaf/0x3740 fs/btrfs/transaction.c:2430

del_balance_item fs/btrfs/volumes.c:3678 [inline]

reset_balance_state+0x25e/0x3c0 fs/btrfs/volumes.c:3742

btrfs_balance+0xead/0x10c0 fs/btrfs/volumes.c:4574

btrfs_ioctl_balance+0x493/0x7c0 fs/btrfs/ioctl.c:3673

vfs_ioctl fs/ioctl.c:51 [inline]

__do_sys_ioctl fs/ioctl.c:907 [inline]

__se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+0x77/0x7f

[CAUSE]

The allocation failure happens at the start_transaction() inside

prepare_to_relocate(), and during the error handling we call

unset_reloc_control(), which makes fs_info->balance_ctl to be NULL.

Then we continue the error path cleanup in btrfs_balance() by calling

reset_balance_state() which will call del_balance_item() to fully delete

the balance item in the root tree.

However during the small window between set_reloc_contrl() and

unset_reloc_control(), we can have a subvolume tree update and created a

reloc_root for that subvolume.

Then we go into the final btrfs_commit_transaction() of

del_balance_item(), and into btrfs_update_reloc_root() inside

commit_fs_roots().

That function checks if fs_info->reloc_ctl is in the merge_reloc_tree

stage, but since fs_info->reloc_ctl is NULL, it results a NULL pointer

dereference.

[FIX]

Just add extra check on fs_info->reloc_ctl inside

btrfs_update_reloc_root(), before checking

fs_info->reloc_ctl->merge_reloc_tree.

That DEAD_RELOC_TREE handling is to prevent further modification to the

reloc tree during merge stage, but since there is no reloc_ctl at all,

we do not need to bother that.