CVE-2026-43046

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

btrfs: reject root items with drop_progress and zero drop_level

[BUG]

When recovering relocation at mount time, merge_reloc_root() and

btrfs_drop_snapshot() both use BUG_ON(level == 0) to guard against

an impossible state: a non-zero drop_progress combined with a zero

drop_level in a root_item, which can be triggered:

------------[ cut here ]------------

kernel BUG at fs/btrfs/relocation.c:1545!

Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI

CPU: 1 UID: 0 PID: 283 ... Tainted: 6.18.0+ #16 PREEMPT(voluntary)

Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE

Hardware name: QEMU Ubuntu 24.04 PC v2, BIOS 1.16.3-debian-1.16.3-2

RIP: 0010:merge_reloc_root+0x1266/0x1650 fs/btrfs/relocation.c:1545

Code: ffff0000 00004589 d7e9acfa ffffe8a1 79bafebe 02000000

Call Trace:

merge_reloc_roots+0x295/0x890 fs/btrfs/relocation.c:1861

btrfs_recover_relocation+0xd6e/0x11d0 fs/btrfs/relocation.c:4195

btrfs_start_pre_rw_mount+0xa4d/0x1810 fs/btrfs/disk-io.c:3130

open_ctree+0x5824/0x5fe0 fs/btrfs/disk-io.c:3640

btrfs_fill_super fs/btrfs/super.c:987 [inline]

btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]

btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline]

btrfs_get_tree+0x111c/0x2190 fs/btrfs/super.c:2128

vfs_get_tree+0x9a/0x370 fs/super.c:1758

fc_mount fs/namespace.c:1199 [inline]

do_new_mount_fc fs/namespace.c:3642 [inline]

do_new_mount fs/namespace.c:3718 [inline]

path_mount+0x5b8/0x1ea0 fs/namespace.c:4028

do_mount fs/namespace.c:4041 [inline]

__do_sys_mount fs/namespace.c:4229 [inline]

__se_sys_mount fs/namespace.c:4206 [inline]

__x64_sys_mount+0x282/0x320 fs/namespace.c:4206

...

RIP: 0033:0x7f969c9a8fde

Code: 0f1f4000 48c7c2b0 fffffff7 d8648902 b8ffffff ffc3660f

---[ end trace 0000000000000000 ]---

The bug is reproducible on 7.0.0-rc2-next-20260310 with our dynamic

metadata fuzzing tool that corrupts btrfs metadata at runtime.

[CAUSE]

A non-zero drop_progress.objectid means an interrupted

btrfs_drop_snapshot() left a resume point on disk, and in that case

drop_level must be greater than 0 because the checkpoint is only

saved at internal node levels.

Although this invariant is enforced when the kernel writes the root

item, it is not validated when the root item is read back from disk.

That allows on-disk corruption to provide an invalid state with

drop_progress.objectid != 0 and drop_level == 0.

When relocation recovery later processes such a root item,

merge_reloc_root() reads drop_level and hits BUG_ON(level == 0). The

same invalid metadata can also trigger the corresponding BUG_ON() in

btrfs_drop_snapshot().

[FIX]

Fix this by validating the root_item invariant in tree-checker when

reading root items from disk: if drop_progress.objectid is non-zero,

drop_level must also be non-zero. Reject such malformed metadata with

-EUCLEAN before it reaches merge_reloc_root() or btrfs_drop_snapshot()

and triggers the BUG_ON.

After the fix, the same corruption is correctly rejected by tree-checker

and the BUG_ON is no longer triggered.