CVE-2025-39738 (GCVE-0-2025-39738)

Vulnerability from cvelistv5 – Published: 2025-09-11 16:52 – Updated: 2026-08-05 12:04
VLAI
Title
btrfs: do not allow relocation of partially dropped subvolumes
Summary
In the Linux kernel, the following vulnerability has been resolved: btrfs: do not allow relocation of partially dropped subvolumes [BUG] There is an internal report that balance triggered transaction abort, with the following call trace: item 85 key (594509824 169 0) itemoff 12599 itemsize 33 extent refs 1 gen 197740 flags 2 ref#0: tree block backref root 7 item 86 key (594558976 169 0) itemoff 12566 itemsize 33 extent refs 1 gen 197522 flags 2 ref#0: tree block backref root 7 ... BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0 BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117 ------------[ cut here ]------------ BTRFS: Transaction aborted (error -117) WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs] And btrfs check doesn't report anything wrong related to the extent tree. [CAUSE] The cause is a little complex, firstly the extent tree indeed doesn't have the backref for 594526208. The extent tree only have the following two backrefs around that bytenr on-disk: item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33 refs 1 gen 197740 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33 refs 1 gen 197522 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE But the such missing backref item is not an corruption on disk, as the offending delayed ref belongs to subvolume 934, and that subvolume is being dropped: item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439 generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328 last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0 drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2 level 2 generation_v2 198229 And that offending tree block 594526208 is inside the dropped range of that subvolume. That explains why there is no backref item for that bytenr and why btrfs check is not reporting anything wrong. But this also shows another problem, as btrfs will do all the orphan subvolume cleanup at a read-write mount. So half-dropped subvolume should not exist after an RW mount, and balance itself is also exclusive to subvolume cleanup, meaning we shouldn't hit a subvolume half-dropped during relocation. The root cause is, there is no orphan item for this subvolume. In fact there are 5 subvolumes from around 2021 that have the same problem. It looks like the original report has some older kernels running, and caused those zombie subvolumes. Thankfully upstream commit 8d488a8c7ba2 ("btrfs: fix subvolume/snapshot deletion not triggered on mount") has long fixed the bug. [ENHANCEMENT] For repairing such old fs, btrfs-progs will be enhanced. Considering how delayed the problem will show up (at run delayed ref time) and at that time we have to abort transaction already, it is too late. Instead here we reject any half-dropped subvolume for reloc tree at the earliest time, preventing confusion and extra time wasted on debugging similar bugs.
Severity
7.3 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < fa086b1398cf7e5f7dee7241bd5f2855cb5df8dc (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < fcb1f77b8ed8795608ca7a1f6505e2b07236c1f3 (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < f83d4c81bda3b7d1813268ab77408f7a0ce691ff (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < 39a93e1c9dbf7e11632efeb20fcf0fc1dcf64d51 (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < 125e94a4b76b7b75d194f85bedd628097d2121f0 (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < 4e403bd8e127d40dc7c05f06ee969c1ba1537ec5 (git)
Affected: 638331fa56caeaa8b4d31cc1dfbe0ce989bcff67 , < 4289b494ac553e74e86fed1c66b2bf9530bc1082 (git)
Create a notification for this product.
Linux Linux Affected: 5.11
Unaffected: 0 , < 5.11 (semver)
Unaffected: 5.15.190 , ≤ 5.15.* (semver)
Unaffected: 6.1.149 , ≤ 6.1.* (semver)
Unaffected: 6.6.103 , ≤ 6.6.* (semver)
Unaffected: 6.12.43 , ≤ 6.12.* (semver)
Unaffected: 6.15.11 , ≤ 6.15.* (semver)
Unaffected: 6.16.2 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
Create a notification for this product.
Siemens SIMATIC CN 4100 Affected: 0 , < V5.0 (custom)
Create a notification for this product.
Show details on NVD website
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nbtrfs: do not allow relocation of partially dropped subvolumes\n\n[BUG]\nThere is an internal report that balance triggered transaction abort,\nwith the following call trace:\n\n  item 85 key (594509824 169 0) itemoff 12599 itemsize 33\n          extent refs 1 gen 197740 flags 2\n          ref#0: tree block backref root 7\n  item 86 key (594558976 169 0) itemoff 12566 itemsize 33\n          extent refs 1 gen 197522 flags 2\n          ref#0: tree block backref root 7\n ...\n BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0\n BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117\n ------------[ cut here ]------------\n BTRFS: Transaction aborted (error -117)\n WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs]\n\nAnd btrfs check doesn\u0027t report anything wrong related to the extent\ntree.\n\n[CAUSE]\nThe cause is a little complex, firstly the extent tree indeed doesn\u0027t\nhave the backref for 594526208.\n\nThe extent tree only have the following two backrefs around that bytenr\non-disk:\n\n        item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33\n                refs 1 gen 197740 flags TREE_BLOCK\n                tree block skinny level 0\n                (176 0x7) tree block backref root CSUM_TREE\n        item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33\n                refs 1 gen 197522 flags TREE_BLOCK\n                tree block skinny level 0\n                (176 0x7) tree block backref root CSUM_TREE\n\nBut the such missing backref item is not an corruption on disk, as the\noffending delayed ref belongs to subvolume 934, and that subvolume is\nbeing dropped:\n\n        item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439\n                generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328\n                last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0\n                drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2\n                level 2 generation_v2 198229\n\nAnd that offending tree block 594526208 is inside the dropped range of\nthat subvolume.  That explains why there is no backref item for that\nbytenr and why btrfs check is not reporting anything wrong.\n\nBut this also shows another problem, as btrfs will do all the orphan\nsubvolume cleanup at a read-write mount.\n\nSo half-dropped subvolume should not exist after an RW mount, and\nbalance itself is also exclusive to subvolume cleanup, meaning we\nshouldn\u0027t hit a subvolume half-dropped during relocation.\n\nThe root cause is, there is no orphan item for this subvolume.\nIn fact there are 5 subvolumes from around 2021 that have the same\nproblem.\n\nIt looks like the original report has some older kernels running, and\ncaused those zombie subvolumes.\n\nThankfully upstream commit 8d488a8c7ba2 (\"btrfs: fix subvolume/snapshot\ndeletion not triggered on mount\") has long fixed the bug.\n\n[ENHANCEMENT]\nFor repairing such old fs, btrfs-progs will be enhanced.\n\nConsidering how delayed the problem will show up (at run delayed ref\ntime) and at that time we have to abort transaction already, it is too\nlate.\n\nInstead here we reject any half-dropped subvolume for reloc tree at the\nearliest time, preventing confusion and extra time wasted on debugging\nsimilar bugs."
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              "value": "AV:L - The vulnerable path is reached only by mounting a btrfs image containing a partially-dropped subvolume and then relocating a block group (balance/device shrink ioctl or the in-kernel background reclaim worker). There is no network or adjacent exposure \u2014 the attack surface is local filesystem media/ioctl.\nAC:L - An attacker supplying the filesystem image fully controls the required on-disk state (root item with non-zero drop_progress and no orphan item) and can size the fs so that any relocation touches the affected block group; the trigger is deterministic with no race or uncontrolled condition.\nPR:L - Balance nominally gates on CAP_SYS_ADMIN, but an unprivileged local user can introduce the crafted image on typical desktop/NAS deployments via udisks2 loop-setup/removable-media auto-mount, and on zoned btrfs the relocation itself is performed by the kernel\u0027s automatic block-group reclaim with no privileges at all.\nUI:N - No action by a second user is required \u2014 the attacker mounts their own image, and relocation is driven either by the kernel\u0027s own periodic/zoned block-group reclaim worker or by routine scheduled maintenance balance (e.g. btrfsmaintenance), not by an interactive victim action.\nS:U - The damage is confined to the btrfs filesystem and the kernel of the same host; no VM, IOMMU, container, or sandbox security boundary is crossed.\nC:L - Mis-dropped extent references can prematurely free still-referenced extents, producing cross-linked extents where one file\u0027s blocks become readable through another\u0027s, and the failure path dumps extent-tree leaf contents to the kernel log; this is indirect, bounded disclosure rather than an arbitrary read primitive.\nI:H - As the fix states, relocating a half-dropped subvolume makes the kernel attempt to drop references for extents that were already dropped, corrupting extent refcounts and allowing in-use metadata/data extents to be freed and reallocated, which can arbitrarily corrupt other files\u0027 and subvolumes\u0027 on-disk data.\nA:H - The bug reliably produces a WARNING splat in btrfs_run_delayed_refs() and a transaction abort (-EUCLEAN), which forces the entire filesystem read-only for the remaining uptime; on panic_on_warn kernels the WARN escalates to a full panic."
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            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nbtrfs: do not allow relocation of partially dropped subvolumes\n\n[BUG]\nThere is an internal report that balance triggered transaction abort,\nwith the following call trace:\n\n  item 85 key (594509824 169 0) itemoff 12599 itemsize 33\n          extent refs 1 gen 197740 flags 2\n          ref#0: tree block backref root 7\n  item 86 key (594558976 169 0) itemoff 12566 itemsize 33\n          extent refs 1 gen 197522 flags 2\n          ref#0: tree block backref root 7\n ...\n BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0\n BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117\n ------------[ cut here ]------------\n BTRFS: Transaction aborted (error -117)\n WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs]\n\nAnd btrfs check doesn\u0027t report anything wrong related to the extent\ntree.\n\n[CAUSE]\nThe cause is a little complex, firstly the extent tree indeed doesn\u0027t\nhave the backref for 594526208.\n\nThe extent tree only have the following two backrefs around that bytenr\non-disk:\n\n        item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33\n                refs 1 gen 197740 flags TREE_BLOCK\n                tree block skinny level 0\n                (176 0x7) tree block backref root CSUM_TREE\n        item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33\n                refs 1 gen 197522 flags TREE_BLOCK\n                tree block skinny level 0\n                (176 0x7) tree block backref root CSUM_TREE\n\nBut the such missing backref item is not an corruption on disk, as the\noffending delayed ref belongs to subvolume 934, and that subvolume is\nbeing dropped:\n\n        item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439\n                generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328\n                last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0\n                drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2\n                level 2 generation_v2 198229\n\nAnd that offending tree block 594526208 is inside the dropped range of\nthat subvolume.  That explains why there is no backref item for that\nbytenr and why btrfs check is not reporting anything wrong.\n\nBut this also shows another problem, as btrfs will do all the orphan\nsubvolume cleanup at a read-write mount.\n\nSo half-dropped subvolume should not exist after an RW mount, and\nbalance itself is also exclusive to subvolume cleanup, meaning we\nshouldn\u0027t hit a subvolume half-dropped during relocation.\n\nThe root cause is, there is no orphan item for this subvolume.\nIn fact there are 5 subvolumes from around 2021 that have the same\nproblem.\n\nIt looks like the original report has some older kernels running, and\ncaused those zombie subvolumes.\n\nThankfully upstream commit 8d488a8c7ba2 (\"btrfs: fix subvolume/snapshot\ndeletion not triggered on mount\") has long fixed the bug.\n\n[ENHANCEMENT]\nFor repairing such old fs, btrfs-progs will be enhanced.\n\nConsidering how delayed the problem will show up (at run delayed ref\ntime) and at that time we have to abort transaction already, it is too\nlate.\n\nInstead here we reject any half-dropped subvolume for reloc tree at the\nearliest time, preventing confusion and extra time wasted on debugging\nsimilar bugs."
          }
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        "id": "CVE-2025-39738",
        "lastModified": "2026-07-30T06:23:43.617",
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                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.3,
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                "confidentialityImpact": "LOW",
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                "scope": "UNCHANGED",
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      "current_release_date": "2026-07-30T08:58:30+00:00",
      "cve": "CVE-2025-39738",
      "id": "CVE-2025-39738",
      "initial_release_date": "2025-09-11T00:00:00+00:00",
      "product_status:known_affected": "42",
      "product_status:known_not_affected": "232",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Linux kernel: Denial of Service in Btrfs due to relocation of partially dropped subvolumes",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-39738.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-10-10T01:59:23Z",
      "cve": "CVE-2025-39738",
      "id": "CVE-2025-39738",
      "initial_release_date": "2025-09-11T23:24:31Z",
      "product_status:known_affected": "791",
      "product_status:recommended": "464",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2025-39738",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2025-39738.json",
      "version": "54"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.