CVE-2024-26727 in Linux
Summary
by MITRE • 04/03/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: do not ASSERT() if the newly created subvolume already got read
[BUG]
There is a syzbot crash, triggered by the ASSERT() during subvolume creation:
assertion failed: !anon_dev, in fs/btrfs/disk-io.c:1319 ------------[ cut here ]------------
kernel BUG at fs/btrfs/disk-io.c:1319! invalid opcode: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:btrfs_get_root_ref.part.0+0x9aa/0xa60 btrfs_get_new_fs_root+0xd3/0xf0 create_subvol+0xd02/0x1650 btrfs_mksubvol+0xe95/0x12b0 __btrfs_ioctl_snap_create+0x2f9/0x4f0 btrfs_ioctl_snap_create+0x16b/0x200 btrfs_ioctl+0x35f0/0x5cf0 __x64_sys_ioctl+0x19d/0x210 do_syscall_64+0x3f/0xe0 entry_SYSCALL_64_after_hwframe+0x63/0x6b ---[ end trace 0000000000000000 ]---
[CAUSE]
During create_subvol(), after inserting root item for the newly created subvolume, we would trigger btrfs_get_new_fs_root() to get the btrfs_root of that subvolume.
The idea here is, we have preallocated an anonymous device number for the subvolume, thus we can assign it to the new subvolume.
But there is really nothing preventing things like backref walk to read the new subvolume. If that happens before we call btrfs_get_new_fs_root(), the subvolume would be read out, with a new anonymous device number assigned already.
In that case, we would trigger ASSERT(), as we really expect no one to read out that subvolume (which is not yet accessible from the fs). But things like backref walk is still possible to trigger the read on the subvolume.
Thus our assumption on the ASSERT() is not correct in the first place.
[FIX]
Fix it by removing the ASSERT(), and just free the @anon_dev, reset it to 0, and continue.
If the subvolume tree is read out by something else, it should have already get a new anon_dev assigned thus we only need to free the preallocated one.
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Analysis
by VulDB Data Team • 08/03/2025
The vulnerability identified as CVE-2024-26727 resides within the Linux kernel's Btrfs file system implementation, specifically in the disk I/O subsystem at fs/btrfs/disk-io.c line 1319. This issue manifests as an assertion failure during subvolume creation operations, leading to a kernel panic and system instability. The problem occurs when the kernel attempts to create a new subvolume and subsequently validates that no anonymous device has been assigned to it. However, the assertion logic fails because concurrent operations such as backreference walks can inadvertently read the newly created subvolume before the creation process completes, thereby assigning it a new anonymous device number.
The technical flaw stems from an incorrect assumption in the kernel's subvolume creation logic where it expects no external processes or internal kernel mechanisms to access the subvolume before it is properly initialized and made available to the file system. This assumption breaks down when background operations like backreference walks occur during the subvolume creation sequence, causing a race condition that triggers the assertion failure. The kernel's design expects that once a subvolume is created, it remains in a state where no external readers can access it until the creation process is complete, but this isolation is not maintained under concurrent access scenarios.
The operational impact of this vulnerability is significant as it can lead to kernel panics and system crashes when Btrfs subvolume creation operations are performed concurrently with other file system operations that might trigger backreference walks or similar read operations. This vulnerability affects systems running Linux kernels with Btrfs file systems and can be exploited by malicious actors or occur during normal system operation when concurrent access patterns trigger the race condition. The instability introduced by this assertion failure can result in complete system downtime and data accessibility issues, particularly in environments where Btrfs subvolumes are frequently created and accessed.
The fix implemented addresses the root cause by removing the problematic assertion and replacing it with proper resource management. Instead of failing the operation when an unexpected read occurs, the kernel now properly frees the preallocated anonymous device number and resets it to zero, allowing the operation to continue. This approach acknowledges that concurrent access during subvolume creation is a legitimate scenario that must be handled gracefully rather than treated as a fatal error. The solution follows established patterns for handling race conditions in kernel code, where assertions are replaced with proper error handling and resource cleanup.
This vulnerability maps to CWE-691, which covers insufficient control flow management, and relates to ATT&CK technique T1490, specifically the exploitation of kernel vulnerabilities for system instability. The fix demonstrates proper kernel development practices by implementing robust error handling instead of relying on assertions that can be violated by legitimate concurrent operations. The solution ensures that Btrfs subvolume creation remains stable even when concurrent access patterns occur, maintaining system reliability and preventing the kernel panic that would otherwise result from the assertion failure.