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

…acntion

[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
---[ end trace 0000000000000000 ]---

[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.

Reported-by: [email protected]
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>

fs/btrfs/relocation.c

@@ -856,7 +856,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
 	btrfs_grab_root(reloc_root);
 
 	/* root->reloc_root will stay until current relocation finished */
-	if (fs_info->reloc_ctl->merge_reloc_tree &&
+	if (fs_info->reloc_ctl && fs_info->reloc_ctl->merge_reloc_tree &&
 	    btrfs_root_refs(root_item) == 0) {
 		set_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
 		/*