CVE-2022-49898 in Linux
Summary
by MITRE • 05/01/2025
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix tree mod log mishandling of reallocated nodes
We have been seeing the following panic in production
kernel BUG at fs/btrfs/tree-mod-log.c:677! invalid opcode: 0000 [#1] SMP
RIP: 0010:tree_mod_log_rewind+0x1b4/0x200 RSP: 0000:ffffc9002c02f890 EFLAGS: 00010293 RAX: 0000000000000003 RBX: ffff8882b448c700 RCX: 0000000000000000 RDX: 0000000000008000 RSI: 00000000000000a7 RDI: ffff88877d831c00 RBP: 0000000000000002 R08: 000000000000009f R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000100c40 R12: 0000000000000001 R13: ffff8886c26d6a00 R14: ffff88829f5424f8 R15: ffff88877d831a00 FS: 00007fee1d80c780(0000) GS:ffff8890400c0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fee1963a020 CR3: 0000000434f33002 CR4: 00000000007706e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 PKRU: 55555554 Call Trace: btrfs_get_old_root+0x12b/0x420 btrfs_search_old_slot+0x64/0x2f0 ? tree_mod_log_oldest_root+0x3d/0xf0 resolve_indirect_ref+0xfd/0x660 ? ulist_alloc+0x31/0x60 ? kmem_cache_alloc_trace+0x114/0x2c0 find_parent_nodes+0x97a/0x17e0 ? ulist_alloc+0x30/0x60 btrfs_find_all_roots_safe+0x97/0x150 iterate_extent_inodes+0x154/0x370 ? btrfs_search_path_in_tree+0x240/0x240 iterate_inodes_from_logical+0x98/0xd0 ? btrfs_search_path_in_tree+0x240/0x240 btrfs_ioctl_logical_to_ino+0xd9/0x180 btrfs_ioctl+0xe2/0x2ec0 ? __mod_memcg_lruvec_state+0x3d/0x280 ? do_sys_openat2+0x6d/0x140 ? kretprobe_dispatcher+0x47/0x70 ? kretprobe_rethook_handler+0x38/0x50 ? rethook_trampoline_handler+0x82/0x140 ? arch_rethook_trampoline_callback+0x3b/0x50 ? kmem_cache_free+0xfb/0x270 ? do_sys_openat2+0xd5/0x140 __x64_sys_ioctl+0x71/0xb0 do_syscall_64+0x2d/0x40
Which is this code in tree_mod_log_rewind()
switch (tm->op) {
case BTRFS_MOD_LOG_KEY_REMOVE_WHILE_FREEING: BUG_ON(tm->slot < n);
This occurs because we replay the nodes in order that they happened, and when we do a REPLACE we will log a REMOVE_WHILE_FREEING for every slot, starting at 0. 'n' here is the number of items in this block, which in this case was 1, but we had 2 REMOVE_WHILE_FREEING operations.
The actual root cause of this was that we were replaying operations for a block that shouldn't have been replayed. Consider the following sequence of events
1. We have an already modified root, and we do a btrfs_get_tree_mod_seq(). 2. We begin removing items from this root, triggering KEY_REPLACE for it's child slots. 3. We remove one of the 2 children this root node points to, thus triggering the root node promotion of the remaining child, and freeing this node. 4. We modify a new root, and re-allocate the above node to the root node of this other root.
The tree mod log looks something like this
logical 0 op KEY_REPLACE (slot 1) seq 2 logical 0 op KEY_REMOVE (slot 1) seq 3 logical 0 op KEY_REMOVE_WHILE_FREEING (slot 0) seq 4 logical 4096 op LOG_ROOT_REPLACE (old logical 0) seq 5 logical 8192 op KEY_REMOVE_WHILE_FREEING (slot 1) seq 6 logical 8192 op KEY_REMOVE_WHILE_FREEING (slot 0) seq 7 logical 0 op LOG_ROOT_REPLACE (old logical 8192) seq 8
>From here the bug is triggered by the following steps
1. Call btrfs_get_old_root() on the new_root. 2. We call tree_mod_log_oldest_root(btrfs_root_node(new_root)), which is currently logical 0. 3. tree_mod_log_oldest_root() calls tree_mod_log_search_oldest(), which gives us the KEY_REPLACE seq 2, and since that's not a LOG_ROOT_REPLACE we incorrectly believe that we don't have an old root, because we expect that the most recent change should be a LOG_ROOT_REPLACE. 4. Back in tree_mod_log_oldest_root() we don't have a LOG_ROOT_REPLACE, so we don't set old_root, we simply use our e ---truncated---
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Analysis
by VulDB Data Team • 11/11/2025
The vulnerability described in CVE-2022-49898 affects the Linux kernel's Btrfs file system implementation, specifically within the tree modification logging subsystem. This issue manifests as a kernel panic due to improper handling of reallocated nodes during tree modification log replay operations. The flaw occurs in the fs/btrfs/tree-mod-log.c file at line 677, where an invalid opcode triggers a kernel BUG condition when processing tree modification logs. The panic results from a logical inconsistency in how the system handles REMOVE_WHILE_FREEING operations during node re-allocation scenarios, leading to a scenario where the slot index validation fails during log replay.
The technical root cause involves a race condition and incorrect state management during Btrfs tree operations. When a Btrfs tree undergoes modifications involving node re-allocation, the system maintains a modification log that tracks operations in chronological order. However, during the specific sequence where a node is freed and then re-allocated to a different tree, the system fails to properly distinguish between operations that should be replayed versus those that should be ignored. The bug arises when the system attempts to validate slot indices against the number of items in a block, but encounters a mismatch where the number of REMOVE_WHILE_FREEING operations exceeds the actual item count in the block. This condition is classified under CWE-129 as an "Improper Validation of Array Index" and CWE-691 as "Insufficient Control Flow Management".
The operational impact of this vulnerability is severe, as it can lead to complete system crashes through kernel panics, disrupting service availability and potentially causing data corruption. The vulnerability is triggered during normal Btrfs operations involving logical to inode resolution through the btrfs_ioctl_logical_to_ino function, making it exploitable in scenarios where Btrfs file systems are actively used. Attackers could potentially leverage this to cause denial of service attacks against systems running affected kernel versions, though exploitation would require specific conditions related to tree modification sequences and node re-allocation patterns. The vulnerability affects systems using Btrfs file systems with tree modification logging enabled, particularly those performing frequent tree modifications or node re-allocations.
Mitigation strategies include updating to kernel versions that contain the fix for CVE-2022-49898, which addresses the improper handling of reallocated nodes in the tree modification log subsystem. System administrators should prioritize patching affected systems, especially those running Btrfs file systems under heavy modification workloads. Additionally, monitoring for kernel panic events and system instability related to Btrfs operations can help identify potential exploitation attempts. The fix implements proper validation checks to ensure that when replaying tree modification logs, the system correctly identifies when operations should be skipped versus when they should be processed, preventing the invalid slot index validation that leads to the kernel BUG. This aligns with ATT&CK technique T1499.004 for "Endpoint Denial of Service" and addresses the underlying control flow management issues that enable the vulnerability. Organizations should also consider implementing robust kernel monitoring and alerting systems to detect and respond to similar kernel-level anomalies that could indicate exploitation attempts.