CVE-2025-38260 in Linux
Summary
by MITRE • 07/09/2025
In the Linux kernel, the following vulnerability has been resolved:
btrfs: handle csum tree error with rescue=ibadroots correctly
[BUG]
There is syzbot based reproducer that can crash the kernel, with the following call trace: (With some debug output added)
DEBUG: rescue=ibadroots parsed BTRFS: device fsid 14d642db-7b15-43e4-81e6-4b8fac6a25f8 devid 1 transid 8 /dev/loop0 (7:0) scanned by repro (1010) BTRFS info (device loop0): first mount of filesystem 14d642db-7b15-43e4-81e6-4b8fac6a25f8 BTRFS info (device loop0): using blake2b (blake2b-256-generic) checksum algorithm BTRFS info (device loop0): using free-space-tree BTRFS warning (device loop0): checksum verify failed on logical 5312512 mirror 1 wanted 0xb043382657aede36608fd3386d6b001692ff406164733d94e2d9a180412c6003 found 0x810ceb2bacb7f0f9eb2bf3b2b15c02af867cb35ad450898169f3b1f0bd818651 level 0 DEBUG: read tree root path failed for tree csum, ret=-5 BTRFS warning (device loop0): checksum verify failed on logical 5328896 mirror 1 wanted 0x51be4e8b303da58e6340226815b70e3a93592dac3f30dd510c7517454de8567a found 0x51be4e8b303da58e634022a315b70e3a93592dac3f30dd510c7517454de8567a level 0 BTRFS warning (device loop0): checksum verify failed on logical 5292032 mirror 1 wanted 0x1924ccd683be9efc2fa98582ef58760e3848e9043db8649ee382681e220cdee4 found 0x0cb6184f6e8799d9f8cb335dccd1d1832da1071d12290dab3b85b587ecacca6e level 0 process 'repro' launched './file2' with NULL argv: empty string added DEBUG: no csum root, idatacsums=0 ibadroots=134217728 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000041: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000208-0x000000000000020f]
CPU: 5 UID: 0 PID: 1010 Comm: repro Tainted: G OE 6.15.0-custom+ #249 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022 RIP: 0010:btrfs_lookup_csum+0x93/0x3d0 [btrfs]
Call Trace: <TASK> btrfs_lookup_bio_sums+0x47a/0xdf0 [btrfs]
btrfs_submit_bbio+0x43e/0x1a80 [btrfs]
submit_one_bio+0xde/0x160 [btrfs]
btrfs_readahead+0x498/0x6a0 [btrfs]
read_pages+0x1c3/0xb20 page_cache_ra_order+0x4b5/0xc20 filemap_get_pages+0x2d3/0x19e0 filemap_read+0x314/0xde0 __kernel_read+0x35b/0x900 bprm_execve+0x62e/0x1140 do_execveat_common.isra.0+0x3fc/0x520 __x64_sys_execveat+0xdc/0x130 do_syscall_64+0x54/0x1d0 entry_SYSCALL_64_after_hwframe+0x76/0x7e ---[ end trace 0000000000000000 ]---
[CAUSE]
Firstly the fs has a corrupted csum tree root, thus to mount the fs we have to go "ro,rescue=ibadroots" mount option.
Normally with that mount option, a bad csum tree root should set BTRFS_FS_STATE_NO_DATA_CSUMS flag, so that any future data read will ignore csum search.
But in this particular case, we have the following call trace that caused NULL csum root, but not setting BTRFS_FS_STATE_NO_DATA_CSUMS:
load_global_roots_objectid():
ret = btrfs_search_slot(); /* Succeeded */ btrfs_item_key_to_cpu() found = true; /* We found the root item for csum tree. */ root = read_tree_root_path(); if (IS_ERR(root)) {
if (!btrfs_test_opt(fs_info, IGNOREBADROOTS)) /* * Since we have rescue=ibadroots mount option, * @ret is still 0. */ break; if (!found || ret) {
/* @found is true, @ret is 0, error handling for csum * tree is skipped. */ }
This means we completely skipped to set BTRFS_FS_STATE_NO_DATA_CSUMS if the csum tree is corrupted, which results unexpected later csum lookup.
[FIX]
If read_tree_root_path() failed, always populate @ret to the error number.
As at the end of the function, we need @ret to determine if we need to do the extra error handling for csum tree.
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Analysis
by VulDB Data Team • 05/02/2026
The vulnerability described in CVE-2025-38260 affects the Linux kernel's Btrfs filesystem implementation and specifically concerns improper handling of checksum tree errors when the rescue=ibadroots mount option is used. This issue stems from a flawed logic path in the load_global_roots_objectid function that governs how the filesystem manages corrupted checksum trees during mount operations. When a Btrfs filesystem is mounted with rescue=ibadroots, the system should be able to tolerate certain levels of corruption, particularly in metadata structures like checksum trees. However, the bug allows the system to bypass critical error handling mechanisms that would normally flag the filesystem state as corrupted and disable checksum validation for subsequent data operations.
The technical flaw manifests in a conditional logic error within the load_global_roots_objectid function where the return code from read_tree_root_path is not properly propagated when the checksum tree root fails to load. This occurs despite the presence of the rescue=ibadroots mount option which is explicitly designed to handle such scenarios by allowing filesystem mounting even with corrupted metadata. The system correctly identifies that a checksum tree is corrupted but fails to set the BTRFS_FS_STATE_NO_DATA_CSUMS flag, which is essential for preventing further checksum verification attempts on data blocks. This oversight leads to a scenario where the system attempts to perform checksum lookups against a NULL checksum root, resulting in a null pointer dereference and ultimately a kernel panic.
The operational impact of this vulnerability is severe as it can lead to complete system crashes and denial of service conditions when attempting to access corrupted Btrfs filesystems. The vulnerability is particularly dangerous because it can be triggered through a syzbot-based reproducer that demonstrates the exact conditions leading to the crash. The call trace shows the system attempting to perform a checksum lookup operation through btrfs_lookup_csum which leads to a general protection fault due to accessing an invalid memory address. This represents a classic null pointer dereference vulnerability (CWE-476) that falls under the broader category of improper error handling in kernel space (CWE-252) and can be mapped to ATT&CK technique T1490 (Inhibit System Recovery) as it can prevent normal filesystem operations and system recovery mechanisms from functioning properly. The vulnerability essentially creates a situation where a filesystem that should be recoverable through the rescue=ibadroots mechanism becomes completely unusable due to the kernel crash.
The fix for this vulnerability involves ensuring that when read_tree_root_path() fails to load the checksum tree root, the error code is properly propagated through the @ret variable. This ensures that the subsequent error handling logic correctly identifies the corrupted state and sets the appropriate flags to disable checksum validation for the remainder of the filesystem session. The fix aligns with security best practices for error handling in kernel modules and follows the principle of least privilege by preventing further operations on corrupted metadata structures. This mitigation ensures that filesystems mounted with rescue=ibadroots will properly transition to a degraded state where checksum verification is disabled rather than crashing the entire system, thus maintaining system stability while preserving data integrity where possible. The solution addresses the root cause by ensuring proper error propagation and state management in the filesystem's metadata loading logic, preventing the NULL pointer dereference that was causing the kernel panic.