CVE-2025-21754 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix assertion failure when splitting ordered extent after transaction abort
If while we are doing a direct IO write a transaction abort happens, we mark all existing ordered extents with the BTRFS_ORDERED_IOERR flag (done at btrfs_destroy_ordered_extents()), and then after that if we enter btrfs_split_ordered_extent() and the ordered extent has bytes left (meaning we have a bio that doesn't cover the whole ordered extent, see details at btrfs_extract_ordered_extent()), we will fail on the following assertion at btrfs_split_ordered_extent():
ASSERT(!(flags & ~BTRFS_ORDERED_TYPE_FLAGS));
because the BTRFS_ORDERED_IOERR flag is set and the definition of BTRFS_ORDERED_TYPE_FLAGS is just the union of all flags that identify the type of write (regular, nocow, prealloc, compressed, direct IO, encoded).
Fix this by returning an error from btrfs_extract_ordered_extent() if we find the BTRFS_ORDERED_IOERR flag in the ordered extent. The error will be the error that resulted in the transaction abort or -EIO if no transaction abort happened.
This was recently reported by syzbot with the following trace:
FAULT_INJECTION: forcing a failure. name failslab, interval 1, probability 0, space 0, times 1 CPU: 0 UID: 0 PID: 5321 Comm: syz.0.0 Not tainted 6.13.0-rc5-syzkaller #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 Call Trace: <TASK> __dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120 fail_dump lib/fault-inject.c:53 [inline]
should_fail_ex+0x3b0/0x4e0 lib/fault-inject.c:154 should_failslab+0xac/0x100 mm/failslab.c:46 slab_pre_alloc_hook mm/slub.c:4072 [inline]
slab_alloc_node mm/slub.c:4148 [inline]
__do_kmalloc_node mm/slub.c:4297 [inline]
__kmalloc_noprof+0xdd/0x4c0 mm/slub.c:4310 kmalloc_noprof include/linux/slab.h:905 [inline]
kzalloc_noprof include/linux/slab.h:1037 [inline]
btrfs_chunk_alloc_add_chunk_item+0x244/0x1100 fs/btrfs/volumes.c:5742 reserve_chunk_space+0x1ca/0x2c0 fs/btrfs/block-group.c:4292 check_system_chunk fs/btrfs/block-group.c:4319 [inline]
do_chunk_alloc fs/btrfs/block-group.c:3891 [inline]
btrfs_chunk_alloc+0x77b/0xf80 fs/btrfs/block-group.c:4187 find_free_extent_update_loop fs/btrfs/extent-tree.c:4166 [inline]
find_free_extent+0x42d1/0x5810 fs/btrfs/extent-tree.c:4579 btrfs_reserve_extent+0x422/0x810 fs/btrfs/extent-tree.c:4672 btrfs_new_extent_direct fs/btrfs/direct-io.c:186 [inline]
btrfs_get_blocks_direct_write+0x706/0xfa0 fs/btrfs/direct-io.c:321 btrfs_dio_iomap_begin+0xbb7/0x1180 fs/btrfs/direct-io.c:525 iomap_iter+0x697/0xf60 fs/iomap/iter.c:90 __iomap_dio_rw+0xeb9/0x25b0 fs/iomap/direct-io.c:702 btrfs_dio_write fs/btrfs/direct-io.c:775 [inline]
btrfs_direct_write+0x610/0xa30 fs/btrfs/direct-io.c:880 btrfs_do_write_iter+0x2a0/0x760 fs/btrfs/file.c:1397 do_iter_readv_writev+0x600/0x880 vfs_writev+0x376/0xba0 fs/read_write.c:1050 do_pwritev fs/read_write.c:1146 [inline]
__do_sys_pwritev2 fs/read_write.c:1204 [inline]
__se_sys_pwritev2+0x196/0x2b0 fs/read_write.c:1195 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f1281f85d29 RSP: 002b:00007f12819fe038 EFLAGS: 00000246 ORIG_RAX: 0000000000000148 RAX: ffffffffffffffda RBX: 00007f1282176080 RCX: 00007f1281f85d29 RDX: 0000000000000001 RSI: 0000000020000240 RDI: 0000000000000005 RBP: 00007f12819fe090 R08: 0000000000000000 R09: 0000000000000003 R10: 0000000000007000 R11: 0000000000000246 R12: 0000000000000002 R13: 0000000000000000 R14: 00007f1282176080 R15: 00007ffcb9e23328 </TASK> BTRFS error (device loop0 state A): Transaction aborted (error -12) BTRFS: error (device loop0 state A ---truncated---
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21754 resides within the Linux kernel's btrfs file system implementation, specifically in how it handles ordered extents during direct I/O write operations when a transaction abort occurs. This flaw manifests as an assertion failure during the splitting of ordered extents, which can lead to system instability or denial of service. The issue is particularly critical because it involves a core component of the file system's I/O handling mechanism, where errors in transaction management can cascade into broader system failures. When a direct I/O write operation is in progress and a transaction abort is triggered, the system marks existing ordered extents with the BTRFS_ORDERED_IOERR flag to indicate an I/O error condition. However, subsequent processing in the btrfs_split_ordered_extent() function fails due to an assertion that checks flag validity, specifically rejecting the presence of the BTRFS_ORDERED_IOERR flag within the BTRFS_ORDERED_TYPE_FLAGS union, which only includes flags identifying the type of write operation but not error indicators. This mismatch causes the system to crash or enter an inconsistent state.
The root cause of this vulnerability lies in the improper handling of error states within the btrfs file system's ordered extent management logic. During direct I/O operations, when a transaction abort occurs, the system correctly identifies that an error has occurred by setting the BTRFS_ORDERED_IOERR flag. However, the subsequent code path in btrfs_split_ordered_extent() does not account for the possibility that an ordered extent might already be marked with an error flag, leading to a direct assertion failure. This behavior violates fundamental principles of error handling in kernel space, where systems must gracefully manage error conditions rather than crashing on unexpected state transitions. The assertion failure occurs because the code assumes that all ordered extents in the splitting path should only contain type flags, not error flags, which creates an implicit assumption about the state of the system that may not hold during error conditions. This scenario is consistent with CWE-248, an unspecified launch of an exception, and represents a failure to properly handle exceptional conditions in kernel code.
The operational impact of this vulnerability is significant for systems relying on btrfs file systems, particularly those performing direct I/O operations under high load or in environments where transaction failures might occur. The assertion failure can result in immediate system crashes or panic conditions, effectively causing a denial of service. Systems using btrfs for critical storage operations, such as database servers, virtualization platforms, or storage appliances, could experience unexpected downtime or data access failures. The vulnerability is particularly concerning because it can be triggered through standard I/O operations, making it exploitable by any process capable of performing direct I/O writes to a btrfs file system. Additionally, the presence of fault injection mechanisms in the stack trace suggests that this vulnerability could be reproduced under controlled conditions, making it a potential target for exploitation. The error handling mechanism described in the fix, which returns an error from btrfs_extract_ordered_extent() when the BTRFS_ORDERED_IOERR flag is detected, aligns with ATT&CK technique T1489, which involves system denial of service through manipulation of system resources.
The mitigation strategy involves modifying the btrfs file system code to properly handle the error state when ordered extents are encountered with the BTRFS_ORDERED_IOERR flag set. The fix implements a return of an error code from the btrfs_extract_ordered_extent() function when such a flag is detected, ensuring that the system does not proceed to the assertion failure path. This approach follows the principle of graceful degradation, where the system handles error conditions appropriately rather than crashing. The error returned should reflect the original cause of the transaction abort or default to -EIO if no specific error is available. This fix ensures that the system properly propagates I/O errors through the call stack rather than failing with an assertion. The solution also aligns with the broader kernel development practice of ensuring that all code paths properly handle error conditions and that error states are consistently managed throughout the system. The fix is consistent with standard kernel error handling patterns and prevents the assertion failure by avoiding the problematic code path when error conditions are already present in the ordered extent structure. This type of fix is essential for maintaining system stability and preventing cascading failures in complex file system implementations.