CVE-2024-49868

CVE-2024-49868 is a medium-severity vulnerability in Linux Linux Kernel with a CVSS 3.x base score of 5.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-476.

Key facts

Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix a NULL pointer dereference when failed to start a new trasacntion [BUG] Syzbot reported a NULL pointer dereference with the following crash: FAULT_INJECTION: forcing a failure. start_transaction+0x830/0x1670 fs/btrfs/transaction.c:676 prepare_to_relocate+0x31f/0x4c0 fs/btrfs/relocation.c:3642 relocate_block_group+0x169/0xd20 fs/btrfs/relocation.c:3678 ... BTRFS info (device loop0): balance: ended with status: -12 Oops: general protection fault, probably for non-canonical address 0xdffffc00000000cc: 0000 [#1] PREEMPT SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000660-0x0000000000000667] RIP: 0010:btrfs_update_reloc_root+0x362/0xa80 fs/btrfs/relocation.c:926 Call Trace: <TASK> commit_fs_roots+0x2ee/0x720 fs/btrfs/transaction.c:1496 btrfs_commit_transaction+0xfaf/0x3740 fs/btrfs/transaction.c:2430 del_balance_item fs/btrfs/volumes.c:3678 [inline] reset_balance_state+0x25e/0x3c0 fs/btrfs/volumes.c:3742 btrfs_balance+0xead/0x10c0 fs/btrfs/volumes.c:4574 btrfs_ioctl_balance+0x493/0x7c0 fs/btrfs/ioctl.c:3673 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:907 [inline] __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893 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 [CAUSE] The allocation failure happens at the start_transaction() inside prepare_to_relocate(), and during the error handling we call unset_reloc_control(), which makes fs_info->balance_ctl to be NULL. Then we continue the error path cleanup in btrfs_balance() by calling reset_balance_state() which will call del_balance_item() to fully delete the balance item in the root tree. However during the small window between set_reloc_contrl() and unset_reloc_control(), we can have a subvolume tree update and created a reloc_root for that subvolume. Then we go into the final btrfs_commit_transaction() of del_balance_item(), and into btrfs_update_reloc_root() inside commit_fs_roots(). That function checks if fs_info->reloc_ctl is in the merge_reloc_tree stage, but since fs_info->reloc_ctl is NULL, it results a NULL pointer dereference. [FIX] Just add extra check on fs_info->reloc_ctl inside btrfs_update_reloc_root(), before checking fs_info->reloc_ctl->merge_reloc_tree. That DEAD_RELOC_TREE handling is to prevent further modification to the reloc tree during merge stage, but since there is no reloc_ctl at all, we do not need to bother that.

Frequently asked questions

What is CVE-2024-49868?
In the Linux kernel, the following vulnerability has been resolved: btrfs: fix a NULL pointer dereference when failed to start a new trasacntion [BUG] Syzbot reported a NULL pointer dereference with the following crash: FAULT_INJECTION: forcing a failure. start_transaction+0x830/0x1670 fs/btrfs/transaction.c:676 prepare_to_relocate+0x31f/0x4c0 fs/btrfs/relocation.c:3642 relocate_block_group+0x169/0xd20 fs/btrfs/relocation.c:3678 ... BTRFS info (device loop0): balance: ended with status: -12 Oops: general protection fault, probably for non-canonical address 0xdffffc00000000cc: 0000 [#1] PREEMPT SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000660-0x0000000000000667] RIP: 0010:btrfs_update_reloc_root+0x362/0xa80 fs/btrfs/relocation.c:926 Call Trace: <TASK> commit_fs_roots+0x2ee/0x720 fs/btrfs/transaction.c:1496 btrfs_commit_transaction+0xfaf/0x3740 fs/btrfs/transaction.c:2430 del_balance_item fs/btrfs/volumes.c:3678 [inline] reset_balance_state+0x25e/0x3c0 fs/btrfs/volumes.c:3742 btrfs_balance+0xead/0x10c0 fs/btrfs/volumes.c:4574 btrfs_ioctl_balance+0x493/0x7c0 fs/btrfs/ioctl.c:3673 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:907 [inline] __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893 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 [CAUSE] The allocation failure happens at the start_transaction() inside prepare_to_relocate(), and during the error handling we call unset_reloc_control(), which makes fs_info->balance_ctl to be NULL. Then we continue the error path cleanup in btrfs_balance() by calling reset_balance_state() which will call del_balance_item() to fully delete the balance item in the root tree. However during the small window between set_reloc_contrl() and unset_reloc_control(), we can have a subvolume tree update and created a reloc_root for that subvolume. Then we go into the final btrfs_commit_transaction() of del_balance_item(), and into btrfs_update_reloc_root() inside commit_fs_roots(). That function checks if fs_info->reloc_ctl is in the merge_reloc_tree stage, but since fs_info->reloc_ctl is NULL, it results a NULL pointer dereference. [FIX] Just add extra check on fs_info->reloc_ctl inside btrfs_update_reloc_root(), before checking fs_info->reloc_ctl->merge_reloc_tree. That DEAD_RELOC_TREE handling is to prevent further modification to the reloc tree during merge stage, but since there is no reloc_ctl at all, we do not need to bother that.
How severe is CVE-2024-49868?
CVE-2024-49868 has a CVSS 3.x base score of 5.5, rated medium severity. It is exploitable over local access with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2024-49868 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (21st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2024-49868?
CVE-2024-49868 affects Linux Linux Kernel. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2024-49868?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2024-49868 have an EU (EUVD) identifier?
Yes. CVE-2024-49868 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2024-43781.
When was CVE-2024-49868 published?
CVE-2024-49868 was published on 2024-10-21 and last updated on 2026-06-17.

References

Affected products (1)

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