Bug 2402283 (CVE-2023-53618)

Summary: CVE-2023-53618 kernel: btrfs: reject invalid reloc tree root keys with stack dump
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A file-system validation flaw was found in the Linux kernel B-tree File System (Btrfs) in the way relocation tree keys are accepted. Crafted on-disk data could present a relocation tree for a non-subvolume tree, leading to internal assertions and a crash during tree merging. A local user mounting or accessing a crafted image could use this flaw to crash the system, resulting in a denial of service.
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Description OSIDB Bzimport 2025-10-07 16:07:14 UTC 
In the Linux kernel, the following vulnerability has been resolved:

btrfs: reject invalid reloc tree root keys with stack dump

[BUG]
Syzbot reported a crash that an ASSERT() got triggered inside
prepare_to_merge().

That ASSERT() makes sure the reloc tree is properly pointed back by its
subvolume tree.

[CAUSE]
After more debugging output, it turns out we had an invalid reloc tree:

  BTRFS error (device loop1): reloc tree mismatch, root 8 has no reloc root, expect reloc root key (-8, 132, 8) gen 17

Note the above root key is (TREE_RELOC_OBJECTID, ROOT_ITEM,
QUOTA_TREE_OBJECTID), meaning it's a reloc tree for quota tree.

But reloc trees can only exist for subvolumes, as for non-subvolume
trees, we just COW the involved tree block, no need to create a reloc
tree since those tree blocks won't be shared with other trees.

Only subvolumes tree can share tree blocks with other trees (thus they
have BTRFS_ROOT_SHAREABLE flag).

Thus this new debug output proves my previous assumption that corrupted
on-disk data can trigger that ASSERT().

[FIX]
Besides the dedicated fix and the graceful exit, also let tree-checker to
check such root keys, to make sure reloc trees can only exist for subvolumes.
Comment 1 Jon Moroney 2025-10-07 20:01:56 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025100709-CVE-2023-53618-7074@gregkh/T