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CVE-2026-89557
md: do overflow check for sb->bblog_shift in super_1_load()
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem drivers/md. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89557 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
In super_1_load(), sb->bblog_shift is an __u8 type value loaded from on- disk superblock. It is used for badblocks API badblocks_set() by the following sequence, 1930 rdev->badblocks.shift = sb->bblog_shift; 1931 for (i = 0 ; i < (sectors << (9-3)) ; i++, bbp++) { 1932 u64 bb = le64_to_cpu(*bbp); 1933 int count = bb & (0x3ff); 1934 u64 sector = bb >> 10; 1935 sector <<= sb->bblog_shift; 1936 count <<= sb->bblog_shift; 1937 if (bb + 1 == 0) 1938 break; 1939 if (!badblocks_set(&rdev->badblocks, sector, count, 1)) 1940 return -EINVAL; 1941 } bb->bblog_shit is in range of 0-255, variable sector is 64bit width, for an invalid bb->bblog_shit, it is possible to make sector be overflowed by the following calculation, 1935 sector <<= sb->bblog_shift; Then in turn when call badblocks_set() at line 1939 with the invalid rdev->badblocks.shift set at line 1930, may result an overflow inside _badblocks_clear() in block/badblocks.c. Although there are many places to call badblocks APIs, the non-zero shift value is only used in super_1_load(), other places always use 0 as the shift value. Therefore it is unnecessary to do a general shift value overflow check inside badblock API, and just check here as the caller. This may avoid unnecessary check, make the badblocks API code more simple and elegant.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/md/md.c
- What this is
- SOFTWARE RAID (Multiple Disks) SUPPORTMAINTAINERS section matched on drivers/md/md* · linux-raid@vger.kernel.org · Supported
- Kernel module
- md_mod
Check it on the machine
grep -wE 'CONFIG_BLK_DEV_MD|CONFIG_MD' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w md_modPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_BLK_DEV_MD, CONFIG_MD
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-09-11 (37 days ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 5.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/md
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 3.1 | 5.10.270 | 94c820d99a43 |
| 3.1 | 5.15.221 | 573fb68105fd |
| 3.1 | 6.1.188 | 323f3a056dbc |
| 3.1 | 6.6.157 | 0a03f9541c06 |
| 3.1 | 6.12.109 | 75d15738fd33 |
| 3.1 | 6.18.50 | 3b097416b4cf |
| 3.1 | 7.2.4 | df7d4d011d5a |
| 3.1 | 7.3 | 35d522bd3246 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- super_1_load() parses an MD v1 superblock and bad-block log from a local block device via md_import_device() (ADD_NEW_DISK ioctl, sysfs new_dev, udev/mdadm incremental, or boot autodetect), not a network protocol handler such as ksmbd, nfsd, or in-kernel TCP.
- AC:LAttack complexity, low
- The attacker fully controls on-disk bblog_shift (an unchecked __u8, 0-255) and the bad-block log and can set a valid sb_csum; importing that RAID member deterministically overflows sector/count shifts and badblocks rounding with no race or attacker-uncontrollable layout.
- PR:LPrivileges required, low
- md ioctls/sysfs require CAP_SYS_ADMIN, but an unprivileged local user can present a crafted RAID member (udisks loop-setup or equivalent); default udev mdadm --incremental then runs as root and the kernel parses the superblock, so init-namespace root is not required of the attacker.
- UI:NUser interaction, none
- The attacker attaches and triggers parse of their own crafted image (loop/udisks) themselves; automated udev/mdadm incremental assembly does not require a separate victim to open a file or confirm a prompt.
- S:UScope, unchanged
- Impact stays in the host kernel MD/badblocks code and that system's storage; this is ordinary same-host kernel memory corruption or denial of service, not a VM escape, IOMMU bypass, or other cross-authority boundary crossing.
- C:HConfidentiality impact, high
- Overflowed bblog_shift poisons rdev->badblocks.shift and BB_MAKE() encodings in the kernel badblocks table; later badblocks_set/clear/check and raid1/raid10 I/O use those corrupted entries and over-wide shifts, which is kernel memory corruption that can be leveraged for information disclosure.
- I:HIntegrity impact, high
- The same overflow writes corrupted badblocks slots (BB_MAKE with wrapped start/len, including all-ones on zero length) and feeds raid1/raid10 narrow_write_error() a bogus 1<<shift block size, giving kernel-state corruption and mis-sized writes exploitable for integrity and control-flow impact.
- A:HAvailability impact, high
- A crafted shift can divide-by-zero in rounddown/roundup, hit BUG() in front_clear(), hard-lock a CPU in _badblocks_set/_badblocks_clear while holding a seqlock with IRQs off, or oops on undefined over-wide shifts, fully denying availability.