CVE-2021-47072 in Linux
Summary
by MITRE • 03/02/2024
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
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Analysis
by VulDB Data Team • 01/09/2025
The vulnerability CVE-2021-47072 affects the btrfs filesystem implementation within the Linux kernel, specifically addressing a critical inconsistency in directory entry handling during log synchronization operations. This flaw manifests when moving inodes between directories where both the source and destination directories have been previously logged, creating a scenario where stale directory entries persist in the filesystem metadata even after power failure events. The core issue stems from the kernel's failure to properly invalidate directory entries in certain edge cases involving directory index and directory item key ranges, violating fundamental filesystem consistency principles.
The technical root cause lies in the btrfs log replay mechanism's inability to correctly handle directory entry cleanup when the old parent directory's metadata contains keys that fall outside the authoritative range for directory index keys. This situation occurs when directory items and index keys are distributed across different metadata leaf nodes, specifically when the directory item key exists in one leaf while the corresponding directory index key exists in another leaf that is not considered authoritative for the range containing the old dentry. The vulnerability is particularly insidious because it only manifests under very specific conditions involving filesystem layout and key distribution, making it difficult to detect during routine testing.
The operational impact of this vulnerability extends beyond simple data inconsistency, potentially leading to severe filesystem corruption and data loss scenarios. When a power failure occurs after log synchronization but before proper cleanup of stale directory entries, the filesystem may retain references to moved inodes in their original parent directories, creating a state where the same inode appears to exist in multiple locations simultaneously. This condition violates the fundamental principle of filesystem integrity and can cause applications to access incorrect data or fail with unexpected errors. The vulnerability affects systems using btrfs filesystems with specific block layout configurations, particularly those with large leaf sizes and specific inode distribution patterns that trigger the edge case in the logging code path.
Security implications of this vulnerability align with CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer and CWE-121 Stack-based Buffer Overflow, as the issue involves improper handling of filesystem metadata structures that could potentially be exploited to manipulate directory entry references. The vulnerability also maps to ATT&CK technique T1070.004 Indicator Removal on Host, as the improper cleanup of directory entries could mask malicious activities or create false data trails. Mitigation strategies should focus on immediate kernel updates to patch the btrfs implementation, combined with filesystem consistency checks and regular backup verification procedures. System administrators should also implement monitoring for filesystem corruption indicators and consider filesystem-specific recovery procedures that account for potential stale directory entry conditions. The fix requires careful handling of directory metadata synchronization during log replay operations and proper invalidation of directory entries that no longer represent valid filesystem state after inode movement operations.