[PATCH] btrfs: reject global extent/csum roots without offset 0 when extent_tree_v2 is off

[ZhengYuan Huang <gality369@gmail.com>]

Without EXTENT_TREE_V2, btrfs_extent_root() and btrfs_csum_root() always
look up the global roots at offset 0. A crafted image can provide only
non-zero offsets for the extent/csum global roots, so the offset 0 lookup
returns NULL and later leads to a NULL dereference
(e.g. in backup_super_roots()).

Fix this by detecting this at mount time: when loading extent/csum
global roots without EXTENT_TREE_V2, require that an offset 0 root item
exists, otherwise fail the mount with -EUCLEAN.

Tested with a crafted image that has only non-zero offset global roots,
which triggers the KASAN null-ptr-deref in backup_super_roots() before
the fix, and fails the mount with -EUCLEAN after the fix.

Fixes: f7238e509404 ("btrfs: add support for multiple global roots")
Cc: stable@vger.kernel.org # v5.18+
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
---
A KASAN null-ptr-deref was triggered when mounting a crafted btrfs
image and then doing a simple write/rename workload (e.g. moving any
file into the mountpoint). The crash happens later during a transaction
commit, in the super backup roots path.

Root cause
==========

backup_super_roots() looks up both the extent and csum roots through
the "global roots" rb-tree:

extent_root = btrfs_extent_root(fs_info, 0);
csum_root = btrfs_csum_root(fs_info, 0);

and then unconditionally dereferences extent_root->node and
csum_root->node.

Both btrfs_extent_root() and btrfs_csum_root() build a key with:

.offset = btrfs_global_root_id(fs_info, bytenr)

and btrfs_global_root_id() returns 0 when the filesystem does not have
the EXTENT_TREE_V2 incompat feature. This means that for
non-extent_tree_v2 filesystems, the implementation assumes that there is
always a global root item for:

(BTRFS_EXTENT_TREE_OBJECTID, BTRFS_ROOT_ITEM_KEY, offset=0)
(BTRFS_CSUM_TREE_OBJECTID, BTRFS_ROOT_ITEM_KEY, offset=0)

However, an image without an offset = 0 root item can still be
successfully mounted. The mount-time validation logic for the csum tree
resides in load_global_roots_objectid(). Currently,
load_global_roots_objectid() loads whatever root items exist for
a given objectid, but it does not validate that offset = 0
exists when EXTENT_TREE_V2 is not enabled. With a crafted image,
the first (and only) root item for the csum tree (and similarly for
extent) can have a large non-zero offset (e.g. 0x10000000000).
load_global_roots_objectid() inserts that root into the rb-tree, but
there is no entry for offset 0. Later, btrfs_csum_root(..., 0) looks up
offset 0, returns NULL, and backup_super_roots() dereferences it,
causing the crash.

So the bug is an invariant mismatch:

- lookup side (btrfs_{extent,csum}_root) assumes offset=0 exists when
extent_tree_v2 is off

- load side (load_global_roots_objectid) does not enforce that assumption

Fix
===
Teach load_global_roots_objectid() to enforce the "offset 0 must exist"
invariant for the extent and csum trees when EXTENT_TREE_V2 is not enabled.
If at least one root item is found but none has offset 0, fail the mount
with -EUCLEAN and emit an explicit error message. This prevents later
NULL dereferences and also avoids propagating a corrupted/unsupported
global-root layout into runtime.

Note: extent_root is subject to the same assumption and is fixed by the
same check (backup_super_roots dereferences extent_root->node as well).

KASAN report
============
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 0 UID: 0 PID: 34 Comm: kworker/u8:1 Tainted: G           OE       6.18.0 #1 PREEMPT(voluntary)
Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: events_unbound btrfs_async_reclaim_data_space
RIP: 0010:backup_super_roots fs/btrfs/disk-io.c:1680 [inline]
RIP: 0010:write_all_supers+0x2d56/0x4980 fs/btrfs/disk-io.c:4022
Code: 40 38 f2 7f 08 84 d2 0f 85 72 19 00 00 49 8d 7c 24 18 41 88 9e 9a 00 00 00 48 ba 00 00 00 00 00 fc ff df 48 89 fe 48 c1 ee 03 <80> 3c 16 00 0f 85 33 19 00 00 48 ba 00 00 00 00 00 fc ff df 49 8b
RSP: 0018:ffff88800a617758 EFLAGS: 00010216
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: dffffc0000000000 RSI: 0000000000000003 RDI: 0000000000000018
RBP: ffff88800a617898 R08: 1ffff110023f53a8 R09: 0000000000000000
R10: ffff8880173a8000 R11: 0000000000000001 R12: 0000000000000000
R13: ffff888000000000 R14: ffff88800b676d23 R15: ffffea0000000000
FS:  0000000000000000(0000) GS:ffff8880e4d3e000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005baa3c26f678 CR3: 000000000e9b8000 CR4: 00000000000006f0
Call Trace:
<TASK>
? __pfx_write_all_supers+0x10/0x10 fs/btrfs/disk-io.c:3980
? btrfs_commit_transaction+0x2818/0x3d90 fs/btrfs/transaction.c:2526
? __lock_release kernel/locking/lockdep.c:5574 [inline]
? lock_release+0x122/0x2a0 kernel/locking/lockdep.c:5889
btrfs_commit_transaction+0x28cc/0x3d90 fs/btrfs/transaction.c:2541
? __pfx_btrfs_commit_transaction+0x10/0x10 fs/btrfs/transaction.c:1919
? start_transaction+0x244/0x1740 fs/btrfs/transaction.c:780
btrfs_commit_current_transaction+0x55/0xf0 fs/btrfs/transaction.c:2010
flush_space+0x7d1/0xc40 fs/btrfs/space-info.c:888
? mark_usage kernel/locking/lockdep.c:4674 [inline]
? __lock_acquire+0x43e/0x21e0 kernel/locking/lockdep.c:5191
? __pfx_flush_space+0x10/0x10 include/trace/events/btrfs.h:2362
? instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
? atomic_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:1301 [inline]
? queued_spin_lock include/asm-generic/qspinlock.h:111 [inline]
? do_raw_spin_lock+0x133/0x290 kernel/locking/spinlock_debug.c:116
? find_held_lock+0x31/0x90 kernel/locking/lockdep.c:5350
? spin_unlock include/linux/spinlock.h:391 [inline]
? do_async_reclaim_data_space+0x369/0x5e0 fs/btrfs/space-info.c:1441
? __kasan_check_read+0x11/0x20 mm/kasan/shadow.c:31
? instrument_atomic_read include/linux/instrumented.h:68 [inline]
? atomic_read include/linux/atomic/atomic-instrumented.h:32 [inline]
? queued_spin_is_locked include/asm-generic/qspinlock.h:57 [inline]
? debug_spin_unlock kernel/locking/spinlock_debug.c:101 [inline]
? do_raw_spin_unlock+0x59/0x200 kernel/locking/spinlock_debug.c:141
do_async_reclaim_data_space+0x3c1/0x5e0 fs/btrfs/space-info.c:1410
btrfs_async_reclaim_data_space+0x3f/0xb0 fs/btrfs/space-info.c:1458
process_one_work+0x8e0/0x1980 kernel/workqueue.c:3263
? __pfx_process_one_work+0x10/0x10 include/linux/list.h:226
? move_linked_works+0x1a8/0x2c0 kernel/workqueue.c:1165
? assign_work+0x19d/0x240 kernel/workqueue.c:1206
? __lock_is_held kernel/locking/lockdep.c:5601 [inline]
? lock_is_held_type+0xa3/0x130 kernel/locking/lockdep.c:5940
process_scheduled_works kernel/workqueue.c:3346 [inline]
worker_thread+0x683/0xf80 kernel/workqueue.c:3427
? __pfx_worker_thread+0x10/0x10 kernel/workqueue.c:3570
kthread+0x3f0/0x850 kernel/kthread.c:463
? __pfx_kthread+0x10/0x10 arch/x86/include/asm/bitops.h:202
? trace_hardirqs_on+0x53/0x60 kernel/trace/trace_preemptirq.c:79
? __raw_spin_unlock_irq include/linux/spinlock_api_smp.h:159 [inline]
? _raw_spin_unlock_irq+0x27/0x70 kernel/locking/spinlock.c:202
? spin_unlock_irq include/linux/spinlock.h:401 [inline]
? calculate_sigpending+0x7c/0xb0 kernel/signal.c:194
? __pfx_kthread+0x10/0x10 arch/x86/include/asm/bitops.h:202
ret_from_fork+0x50f/0x610 arch/x86/kernel/process.c:158
? __pfx_kthread+0x10/0x10 arch/x86/include/asm/bitops.h:202
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:backup_super_roots fs/btrfs/disk-io.c:1680 [inline]
RIP: 0010:write_all_supers+0x2d56/0x4980 fs/btrfs/disk-io.c:4022
Code: 40 38 f2 7f 08 84 d2 0f 85 72 19 00 00 49 8d 7c 24 18 41 88 9e 9a 00 00 00 48 ba 00 00 00 00 00 fc ff df 48 89 fe 48 c1 ee 03 <80> 3c 16 00 0f 85 33 19 00 00 48 ba 00 00 00 00 00 fc ff df 49 8b
RSP: 0018:ffff88800a617758 EFLAGS: 00010216
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: dffffc0000000000 RSI: 0000000000000003 RDI: 0000000000000018
RBP: ffff88800a617898 R08: 1ffff110023f53a8 R09: 0000000000000000
R10: ffff8880173a8000 R11: 0000000000000001 R12: 0000000000000000
R13: ffff888000000000 R14: ffff88800b676d23 R15: ffffea0000000000
FS:  0000000000000000(0000) GS:ffff8880e4d3e000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005baa3c26f678 CR3: 000000000e9b8000 CR4: 00000000000006f0
----------------
Code disassembly (best guess):
   0:	40 38 f2             	cmp    %sil,%dl
   3:	7f 08                	jg     0xd
   5:	84 d2                	test   %dl,%dl
   7:	0f 85 72 19 00 00    	jne    0x197f
   d:	49 8d 7c 24 18       	lea    0x18(%r12),%rdi
  12:	41 88 9e 9a 00 00 00 	mov    %bl,0x9a(%r14)
  19:	48 ba 00 00 00 00 00 	movabs $0xdffffc0000000000,%rdx
  20:	fc ff df
  23:	48 89 fe             	mov    %rdi,%rsi
  26:	48 c1 ee 03          	shr    $0x3,%rsi
* 2a:	80 3c 16 00          	cmpb   $0x0,(%rsi,%rdx,1) <-- trapping instruction
  2e:	0f 85 33 19 00 00    	jne    0x1967
  34:	48 ba 00 00 00 00 00 	movabs $0xdffffc0000000000,%rdx
  3b:	fc ff df
  3e:	49                   	rex.WB
  3f:	8b                   	.byte 0x8b

Reproduction (v6.18, x86_64, KASAN)
===================================
1. Download the crafted image (tested with Linux v6.18 + KASAN):
  https://drive.google.com/file/d/1xV0pjI3N-D83IzH62dphWCAD0RR-sV4U/view
2. Attach it as a block device (example assumes it shows up as /dev/vda),
then mount it read-write:
  mkdir -p /mnt/btrfs
  mount /dev/vda /mnt/btrfs
3. Trigger any metadata update, e.g. move a file into the mountpoint:
  echo test > /tmp/1.txt
  mv /tmp/1.txt /mnt/btrfs/
4. Result:
The system hits a NULL pointer dereference in the commit path, with KASAN
reporting the fault at backup_super_roots() (fs/btrfs/disk-io.c).

Thanks,
ZhengYuan Huang
---
 fs/btrfs/disk-io.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 0aa7e5d1b05f..900e462d8ea1 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2119,6 +2119,18 @@ static int load_global_roots_objectid(struct btrfs_root *tree_root,
 	};
 	bool found = false;
 
+	/*
+	 * Without EXTENT_TREE_V2 we only have a single global extent/csum root.
+	 * btrfs_extent_root() and btrfs_csum_root() always look it up with offset
+	 * 0 (btrfs_global_root_id() returns 0). If we load only non-zero offsets
+	 * here, later users will see NULL and can crash (e.g. when backing up
+	 * super roots during a commit).
+	 */
+	bool need_offset0 = !btrfs_fs_incompat(fs_info, EXTENT_TREE_V2) &&
+		      (objectid == BTRFS_EXTENT_TREE_OBJECTID ||
+		       objectid == BTRFS_CSUM_TREE_OBJECTID);
+	bool found_offset0 = false;
+
 	/* If we have IGNOREDATACSUMS skip loading these roots. */
 	if (objectid == BTRFS_CSUM_TREE_OBJECTID &&
 	    btrfs_test_opt(fs_info, IGNOREDATACSUMS)) {
@@ -2144,6 +2156,8 @@ static int load_global_roots_objectid(struct btrfs_root *tree_root,
 		btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
 		if (key.objectid != objectid)
 			break;
+		if (need_offset0 && key.offset == 0)
+			found_offset0 = true;
 		btrfs_release_path(path);
 
 		/*
@@ -2169,6 +2183,13 @@ static int load_global_roots_objectid(struct btrfs_root *tree_root,
 	}
 	btrfs_release_path(path);
 
+	if (need_offset0 && found && !found_offset0) {
+		btrfs_err(fs_info,
+			  "missing global %s root item with offset 0 (extent_tree_v2 not enabled)",
+			  name);
+		return -EUCLEAN;
+	}
+
 	if (objectid == BTRFS_EXTENT_TREE_OBJECTID)
 		fs_info->nr_global_roots = max_global_id + 1;
 
-- 
2.43.0