CVE-2024-45003 in Linux
Summary
by MITRE • 09/04/2024
In the Linux kernel, the following vulnerability has been resolved:
vfs: Don't evict inode under the inode lru traversing context
The inode reclaiming process(See function prune_icache_sb) collects all reclaimable inodes and mark them with I_FREEING flag at first, at that time, other processes will be stuck if they try getting these inodes (See function find_inode_fast), then the reclaiming process destroy the inodes by function dispose_list(). Some filesystems(eg. ext4 with ea_inode feature, ubifs with xattr) may do inode lookup in the inode evicting callback function, if the inode lookup is operated under the inode lru traversing context, deadlock problems may happen.
Case 1: In function ext4_evict_inode(), the ea inode lookup could happen if ea_inode feature is enabled, the lookup process will be stuck under the evicting context like this:
1. File A has inode i_reg and an ea inode i_ea 2. getfattr(A, xattr_buf) // i_ea is added into lru // lru->i_ea 3. Then, following three processes running like this:
PA PB echo 2 > /proc/sys/vm/drop_caches shrink_slab prune_dcache_sb // i_reg is added into lru, lru->i_ea->i_reg prune_icache_sb list_lru_walk_one inode_lru_isolate i_ea->i_state |= I_FREEING // set inode state inode_lru_isolate __iget(i_reg) spin_unlock(&i_reg->i_lock) spin_unlock(lru_lock) rm file A i_reg->nlink = 0 iput(i_reg) // i_reg->nlink is 0, do evict ext4_evict_inode ext4_xattr_delete_inode ext4_xattr_inode_dec_ref_all ext4_xattr_inode_iget ext4_iget(i_ea->i_ino) iget_locked find_inode_fast __wait_on_freeing_inode(i_ea) ----→ AA deadlock dispose_list // cannot be executed by prune_icache_sb wake_up_bit(&i_ea->i_state)
Case 2: In deleted inode writing function ubifs_jnl_write_inode(), file deleting process holds BASEHD's wbuf->io_mutex while getting the xattr inode, which could race with inode reclaiming process(The reclaiming process could try locking BASEHD's wbuf->io_mutex in inode evicting function), then an ABBA deadlock problem would happen as following:
1. File A has inode ia and a xattr(with inode ixa), regular file B has inode ib and a xattr. 2. getfattr(A, xattr_buf) // ixa is added into lru // lru->ixa 3. Then, following three processes running like this:
PA PB PC echo 2 > /proc/sys/vm/drop_caches shrink_slab prune_dcache_sb // ib and ia are added into lru, lru->ixa->ib->ia prune_icache_sb list_lru_walk_one inode_lru_isolate ixa->i_state |= I_FREEING // set inode state inode_lru_isolate __iget(ib) spin_unlock(&ib->i_lock) spin_unlock(lru_lock) rm file B ib->nlink = 0 rm file A iput(ia) ubifs_evict_inode(ia) ubifs_jnl_delete_inode(ia) ubifs_jnl_write_inode(ia) make_reservation(BASEHD) // Lock wbuf->io_mutex ubifs_iget(ixa->i_ino) iget_locked find_inode_fast __wait_on_freeing_inode(ixa) | iput(ib) // ib->nlink is 0, do evict | ubifs_evict_inode | ubifs_jnl_delete_inode(ib) ↓ ubifs_jnl_write_inode ABBA deadlock ←-----make_reservation(BASEHD) dispose_list // cannot be executed by prune_icache_sb wake_up_bit(&ixa->i_state)
Fix the possible deadlock by using new inode state flag I_LRU_ISOLATING to pin the inode in memory while inode_lru_isolate( ---truncated---
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Analysis
by VulDB Data Team • 10/10/2024
The vulnerability described in CVE-2024-45003 resides within the Linux kernel's virtual file system (VFS) layer, specifically in the inode reclaiming mechanism. This flaw manifests during the process of cleaning up unused inodes from memory, where the kernel attempts to isolate inodes from the LRU (Least Recently Used) list before freeing them. The issue arises when filesystem implementations, such as ext4 and ubifs, perform inode lookups within their eviction callbacks while the system is already traversing the LRU list to identify inodes for reclamation. This creates a potential deadlock scenario as processes wait for resources that are simultaneously being freed, leading to system instability or hangs.
The technical root cause lies in the interaction between the `prune_icache_sb` function, which manages the reclaim of inodes, and the `inode_lru_isolate` function that isolates inodes from the LRU list. When the `I_FREEING` flag is set on an inode, other processes attempting to access that inode are blocked by the `__wait_on_freeing_inode` function. However, if the filesystem's eviction callback triggers an inode lookup during this critical phase, it can lead to a circular wait condition. In the case of ext4, when the `ea_inode` feature is enabled, the `ext4_evict_inode` function may attempt to access extended attribute inodes, which in turn triggers a lookup under the same LRU context, resulting in a deadlock. This corresponds to a CWE-367: Time-of-Check to Time-of-Use (TOCTOU) vulnerability where the state of an inode changes between when it is checked for eviction and when it is actually freed.
The operational impact of this vulnerability extends beyond simple system hangs, potentially affecting the stability of systems under high I/O load or during memory pressure conditions. When the kernel's memory management subsystem becomes deadlocked, it can prevent further inode operations, leading to denial of service conditions for applications relying on file system access. The vulnerability is particularly concerning in environments where extended attributes are frequently used, such as those employing security modules or applications that rely heavily on metadata. The specific deadlock patterns described in the CVE align with ATT&CK technique T1490: Inhibit System Recovery, where system resources are manipulated to prevent normal system recovery operations, effectively causing a denial of service condition.
The fix for this vulnerability introduces a new inode state flag called `I_LRU_ISOLATING`, which serves to pin inodes in memory during the LRU traversal process. This prevents the race condition by ensuring that inodes being isolated for potential eviction cannot be accessed by other processes during the critical phase. This solution addresses the core problem by breaking the circular dependency between the inode reclaiming process and filesystem-specific eviction callbacks. The mitigation strategy effectively implements a form of resource locking that prevents the scenario described in the CVE where inode lookups during eviction could lead to deadlocks. This approach is consistent with security best practices for kernel-level resource management, as it prevents the conditions that lead to system-level deadlocks while maintaining the integrity of the VFS layer's memory management functions. The fix ensures that filesystem-specific operations during inode eviction do not interfere with the core kernel memory management processes, thereby restoring system stability under memory pressure conditions.