CVE-2022-49898: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2022-49898

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---

AI Analysis

Technical Summary

CVE-2022-49898 is a vulnerability in the Linux kernel's Btrfs filesystem implementation, specifically within the tree modification log handling code. The issue arises in the function tree_mod_log_rewind(), which processes logged operations on Btrfs tree nodes during filesystem modifications. The vulnerability is triggered by mishandling of reallocated nodes during replay of tree modification logs, particularly when multiple REMOVE_WHILE_FREEING operations are logged for slots exceeding the number of items in a block. This leads to a kernel BUG triggered by a failed assertion (BUG_ON) in the kernel code, causing a kernel panic and system crash. The root cause is that the replay logic incorrectly processes operations for blocks that should not be replayed, due to an erroneous assumption about the presence of LOG_ROOT_REPLACE operations in the modification sequence. The bug manifests when a root node is promoted and its child nodes are reallocated, causing the tree mod log to contain inconsistent sequences of operations. This inconsistency leads to out-of-bounds slot references during replay, triggering the kernel panic. The vulnerability affects Linux kernel versions containing the faulty Btrfs code prior to the patch. Exploitation requires triggering specific filesystem operations that manipulate Btrfs tree nodes in a particular sequence, which can occur under certain workloads or filesystem states. While no known exploits are reported in the wild, the vulnerability can cause denial of service by crashing the kernel, impacting system availability. No CVSS score is assigned yet, but the vulnerability is significant due to its potential to cause kernel panics and data unavailability on systems using Btrfs.

Potential Impact

For European organizations, the impact of CVE-2022-49898 can be substantial, especially those relying on Linux servers with Btrfs filesystems for critical applications, data storage, or virtualization infrastructure. A kernel panic induced by this vulnerability results in immediate system downtime, potentially causing service interruptions, data loss if unsaved data is in memory, and operational disruption. Organizations in sectors such as finance, healthcare, telecommunications, and government, which often use Linux-based infrastructure, may face increased risk of availability issues. Additionally, recovery from such crashes may require manual intervention, increasing operational costs and downtime. While the vulnerability does not directly lead to privilege escalation or data confidentiality breaches, the denial of service impact on critical systems can indirectly affect business continuity and compliance with data availability regulations such as GDPR. Given the growing adoption of Btrfs in enterprise Linux distributions and cloud environments, the risk surface in Europe is non-trivial.

Mitigation Recommendations

To mitigate CVE-2022-49898, European organizations should: 1) Apply the latest Linux kernel updates that include the patch fixing the tree_mod_log mishandling in Btrfs. Kernel updates from major distributions (e.g., Debian, Ubuntu, Red Hat, SUSE) should be prioritized. 2) Where immediate patching is not feasible, consider migrating critical workloads off Btrfs filesystems to more stable alternatives such as ext4 or XFS until patched kernels are deployed. 3) Implement robust monitoring of kernel logs for early detection of Btrfs-related kernel panics or BUG messages to enable rapid incident response. 4) Conduct thorough testing of Btrfs-dependent applications and workloads in staging environments with updated kernels to ensure stability. 5) Maintain regular backups and disaster recovery plans to minimize data loss and downtime in case of crashes. 6) Limit access to systems with Btrfs filesystems to trusted users and processes to reduce the risk of triggering the vulnerability unintentionally. 7) Engage with Linux distribution vendors and security advisories to stay informed about patches and related vulnerabilities.