Linux Kernel: security vulnerability (CVE-2021-47072)
Published Sources: BSI, CVE.org, EUVD
What is affected, what to do
In the Linux kernel, the following vulnerability has been resolved: btrfs: fix removed dentries still existing after log is synced When we move one inode from one directory to another and both the inode and its previous parent directory were logged before, we are not supposed to have the dentry for the old parent if we have a power failure after the log is synced. Only the new dentry is supposed to exist.
Update Linux Kernel to version 5.12.7, Linux Kernel to version 5.13.
- Affected
- Linux Kernel
lt 5.12..5.12.7
- Linux Kernel
- Fixed in
- Linux Kernel
5.12.7 - Linux Kernel
5.13
- Linux Kernel
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Metrics
5.5
XIEM Effective CVSS
Highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
5.5
CVSS v3
Severity Medium on the scale from 0 to 10.
Source: BSI, CVE.org, EUVD
0.2 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 9.1
Show all metrics
- Severity
- Medium
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 32.67 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 5.5 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 5.5
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.2 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 9.1
- Low — this CVE sits in the lower 10% of all CVEs scored today.
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
- CISA assessment (SSVC)
- Exploitation: none known · not automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 11 March 2024
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
In the Linux kernel, the following vulnerability has been resolved: btrfs: fix removed dentries still existing after log is synced When we move one inode from one directory to another and both the inode and its previous parent directory were logged before, we are not supposed to have the dentry for the old parent if we have a power failure after the log is synced. Only the new dentry is supposed to exist. Generally this works correctly, however there is a scenario where this is not currently working, because the old parent of the file/directory that was moved is not authoritative for a range that includes the dir index and dir item keys of the old dentry. This case is better explained with the following example and reproducer: # The test requires a very specific layout of keys and items in the # fs/subvolume btree to trigger the bug. So we want to make sure that # on whatever platform we are, we have the same leaf/node size. # # Currently in btrfs the node/leaf size can not be smaller than the page # size (but it can be greater than the page size). So use the largest # supported node/leaf size (64K). $ mkfs.btrfs -f -n 65536 /dev/sdc $ mount /dev/sdc /mnt # "testdir" is inode 257. $ mkdir /mnt/testdir $ chmod 755 /mnt/testdir # Create several empty files to have the directory "testdir" with its # items spread over several leaves (7 in this case). $ for ((i = 1; i <= 1200; i++)); do echo -n > /mnt/testdir/file$i done # Create our test directory "dira", inode number 1458, which gets all # its items in leaf 7. # # The BTRFS_DIR_ITEM_KEY item for inode 257 ("testdir") that points to # the entry named "dira" is in leaf 2, while the BTRFS_DIR_INDEX_KEY # item that points to that entry is in leaf 3. # # For this particular filesystem node size (64K), file count and file # names, we endup with the directory entry items from inode 257 in # leaves 2 and 3, as previously mentioned - what matters for triggering # the bug exercised by this test case is that those items are not placed # in leaf 1, they must be placed in a leaf different from the one # containing the inode item for inode 257. # # The corresponding BTRFS_DIR_ITEM_KEY and BTRFS_DIR_INDEX_KEY items for # the parent inode (257) are the following: # # item 460 key (257 DIR_ITEM 3724298081) itemoff 48344 itemsize 34 # location key (1458 INODE_ITEM 0) type DIR # transid 6 data_len 0 name_len 4 # name: dira # # and: # # item 771 key (257 DIR_INDEX 1202) itemoff 36673 itemsize 34 # location key (1458 INODE_ITEM 0) type DIR # transid 6 data_len 0 name_len 4 # name: dira $ mkdir /mnt/testdir/dira # Make sure everything done so far is durably persisted. $ sync # Now do a change to inode 257 ("testdir") that does not result in # COWing leaves 2 and 3 - the leaves that contain the directory items # pointing to inode 1458 (directory "dira"). # # Changing permissions, the owner/group, updating or adding a xattr, # etc, will not change (COW) leaves 2 and 3. So for the sake of # simplicity change the permissions of inode 257, which results in # updating its inode item and therefore change (COW) only leaf 1. $ chmod 700 /mnt/testdir # Now fsync directory inode 257. # # Since only the first leaf was changed/COWed, we log the inode item of # inode 257 and only the dentries found in the first leaf, all have a # key type of BTRFS_DIR_ITEM_KEY, and no keys of type # BTRFS_DIR_INDEX_KEY, because they sort after the former type and none # exist in the first leaf. # # We also log 3 items that represent ranges for dir items and dir # indexes for which the log is authoritative: # # 1) a key of type BTRFS_DIR_LOG_ITEM_KEY, which indicates the log is # authoritative for all BTRFS_DIR_ITEM_KEY keys that have an offset # in the range [0, 2285968570] (the offset here is th ---truncated---
Source: BSI CSAF, BSI WID Portal, cvelistv5, ENISA EUVD, NVD (NIST)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Linux | Linux |
|
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Vendors and products
Affected operating systems
| Linux | linux | linux_kernel | 5.13 |
| Linux | linux | linux_kernel | — |
References & sources
- git.kernel.orgstable/c/6d0924c5b742036b4f20a0ffdf2b6cf3f963f5f6euvd, Patch
- git.kernel.orgstable/c/54a40fc3a1da21b52dbf19f72fdc27a2ec740760euvd, Patch
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-47072"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-47072.svg?lang=en" alt="CVE-2021-47072 – NEOSEC Intel" height="20"></a>