In the Linux kernel, the following vulnerability has been resolved:
hfs: fix missing hfs_bnode_get() in __hfs_bnode_create
Syzbot found a kernel BUG in hfs_bnode_put():
kernel BUG at fs/hfs/bnode.c:466!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 3634 Comm: kworker/u4:5 Not tainted 6.1.0-rc7-syzkaller-00190-g97ee9d1c1696 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Workqueue: writeback wb_workfn (flush-7:0)
RIP: 0010:hfs_bnode_put+0x46f/0x480 fs/hfs/bnode.c:466
Code: 8a 80 ff e9 73 fe ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c a0 fe ff ff 48 89 df e8 db 8a 80 ff e9 93 fe ff ff e8 a1 68 2c ff <0f> 0b e8 9a 68 2c ff 0f 0b 0f 1f 84 00 00 00 00 00 55 41 57 41 56
RSP: 0018:ffffc90003b4f258 EFLAGS: 00010293
RAX: ffffffff825e318f RBX: 0000000000000000 RCX: ffff8880739dd7c0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003b4f430 R08: ffffffff825e2d9b R09: ffffed10045157d1
R10: ffffed10045157d1 R11: 1ffff110045157d0 R12: ffff8880228abe80
R13: ffff88807016c000 R14: dffffc0000000000 R15: ffff8880228abe00
FS: 0000000000000000(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa6ebe88718 CR3: 000000001e93d000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
hfs_write_inode+0x1bc/0xb40
write_inode fs/fs-writeback.c:1440 [inline]
__writeback_single_inode+0x4d6/0x670 fs/fs-writeback.c:1652
writeback_sb_inodes+0xb3b/0x18f0 fs/fs-writeback.c:1878
__writeback_inodes_wb+0x125/0x420 fs/fs-writeback.c:1949
wb_writeback+0x440/0x7b0 fs/fs-writeback.c:2054
wb_check_start_all fs/fs-writeback.c:2176 [inline]
wb_do_writeback fs/fs-writeback.c:2202 [inline]
wb_workfn+0x827/0xef0 fs/fs-writeback.c:2235
process_one_work+0x877/0xdb0 kernel/workqueue.c:2289
worker_thread+0xb14/0x1330 kernel/workqueue.c:2436
kthread+0x266/0x300 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
The BUG_ON() is triggered at here:
/* Dispose of resources used by a node */
void hfs_bnode_put(struct hfs_bnode *node)
{
if (node) {
BUG_ON(!atomic_read(&node->refcnt)); <- we have issue here!!!!
}
}
By tracing the refcnt, I found the node is created by hfs_bmap_alloc()
with refcnt 1. Then the node is used by hfs_btree_write(). There is a
missing of hfs_bnode_get() after find the node. The issue happened in
following path:
hfs_bmap_alloc
hfs_bnode_find
__hfs_bnode_create <- allocate a new node with refcnt 1.
hfs_bnode_put <- decrease the refcnt
hfs_btree_write
hfs_bnode_find
__hfs_bnode_create
hfs_bnode_findhash <- find the node without refcnt increased.
hfs_bnode_put <- trigger the BUG_ON() since refcnt is 0.
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (7.8): severity, escalated by confirmed exploitation. This CVE's 7.8 is its impact severity (EG Severity), not escalated.
- EG Severity
- 7.8 HIGHCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
- EG Likelihood
- 0.2% within 30 daysSource: FIRST EPSS (model v2026.06.15, score date 2026-11-03). FIRST's exploitation probability, not an EchelonGraph model.
- Confirmed exploitation
- None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
- EG Priority
- SSVC Track at medium mission impactTrack at low or medium mission impact; Track* at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.Each input and its source, and whom BOD 26-04 binds: the SSVC and BOD 26-04 blocks below.
SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Track* at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines.
Exploitation none (no CISA value, KEV listing or exploit code on record) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). EchelonGraph holds no SSVC assessment of this CVE from CISA: none of these inputs is from Vulnrichment. EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table
BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.
The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). Where an input reads “CISA's BOD 26-04 default”, no one has published it for this CVE, and CISA's implementation guidance says: “If “Automatable” and “Technical Impact” information items about a CVE ID are not available and the CVE ID is not in the KEV Catalog, CISA will treat its values as “no” and “total,” respectively, until CISA or a third party provides metadata about the CVE ID. CISA will always provide the Vulnrichment data for a CVE ID listed in the KEV Catalog.” Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it