CVE-2021-46982 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
f2fs: compress: fix race condition of overwrite vs truncate
pos_fsstress testcase complains a panic as belew:
------------[ cut here ]------------
kernel BUG at fs/f2fs/compress.c:1082! invalid opcode: 0000 [#1] SMP PTI
CPU: 4 PID: 2753477 Comm: kworker/u16:2 Tainted: G OE 5.12.0-rc1-custom #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014 Workqueue: writeback wb_workfn (flush-252:16) RIP: 0010:prepare_compress_overwrite+0x4c0/0x760 [f2fs]
Call Trace: f2fs_prepare_compress_overwrite+0x5f/0x80 [f2fs]
f2fs_write_cache_pages+0x468/0x8a0 [f2fs]
f2fs_write_data_pages+0x2a4/0x2f0 [f2fs]
do_writepages+0x38/0xc0 __writeback_single_inode+0x44/0x2a0 writeback_sb_inodes+0x223/0x4d0 __writeback_inodes_wb+0x56/0xf0 wb_writeback+0x1dd/0x290 wb_workfn+0x309/0x500 process_one_work+0x220/0x3c0 worker_thread+0x53/0x420 kthread+0x12f/0x150 ret_from_fork+0x22/0x30
The root cause is truncate() may race with overwrite as below, so that one reference count left in page can not guarantee the page attaching in mapping tree all the time, after truncation, later find_lock_page() may return NULL pointer.
- prepare_compress_overwrite - f2fs_pagecache_get_page - unlock_page - f2fs_setattr - truncate_setsize - truncate_inode_page - delete_from_page_cache - find_lock_page
Fix this by avoiding referencing updated page.
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Analysis
by VulDB Data Team • 12/31/2024
The vulnerability CVE-2021-46982 affects the Linux kernel's f2fs filesystem implementation and represents a race condition between the truncate and overwrite operations during compressed data handling. This issue manifests as a kernel panic due to an invalid opcode when the kernel attempts to access memory that has been freed or improperly managed during concurrent file operations. The problem specifically occurs in the compress.c module at line 1082, where the kernel encounters a critical failure during the prepare_compress_overwrite function execution. The vulnerability is particularly concerning because it can lead to system instability and potential denial of service conditions when multiple processes attempt to simultaneously truncate and overwrite compressed files within the same filesystem.
The technical root cause involves a classic race condition scenario where the truncate operation can occur between the page cache lookup and the actual page modification phases. During normal operation, the prepare_compress_overwrite function retrieves pages from the page cache using f2fs_pagecache_get_page, unlocks them, and then proceeds with compression operations. However, if a truncate operation intervenes during this window, it can cause the page to be removed from the page cache through delete_from_page_cache operations triggered by truncate_inode_page. This leaves the system with a dangling reference to a page that may no longer exist in the mapping tree, causing find_lock_page to return NULL and ultimately leading to the kernel BUG and system panic. The call trace demonstrates this sequence clearly, showing how the race condition unfolds through the kernel's writeback subsystem and page cache management functions.
This vulnerability directly relates to CWE-362, which describes a race condition where two or more threads can access shared data concurrently, leading to unpredictable behavior. The issue also maps to ATT&CK technique T1499.001, which involves unauthorized modification of files, as the race condition could potentially be exploited to corrupt file data or create inconsistent filesystem states. The operational impact extends beyond simple system crashes, as this vulnerability affects the reliability of compressed file operations in f2fs filesystems, particularly in environments where high concurrency and rapid file modifications occur. Systems utilizing f2fs with compression enabled are at risk when running workloads that frequently perform truncate and overwrite operations on compressed files, making this a significant concern for storage systems and servers handling large volumes of dynamic content.
The fix for this vulnerability involves modifying the code to avoid referencing pages that may have been updated or removed during the race window. The solution requires implementing proper synchronization mechanisms or reordering operations to ensure that page references remain valid throughout the compression process. This approach prevents the kernel from attempting to access freed memory and maintains the integrity of the page cache management during concurrent operations. System administrators should prioritize applying the kernel patches that address this vulnerability, particularly in production environments where f2fs filesystems with compression are actively used. Additionally, monitoring for kernel panic events and implementing proper system logging can help detect potential exploitation attempts or identify systems that may be vulnerable to similar race condition scenarios in other filesystem components.