Look up another CVE · open CVE-2026-80537 in the interactive tracker
CVE-2026-80537
xfs: fix off-by-one in rtrefcount btree root level validation
Published 2026-08-26 (2 months ago) by the Linux kernel CNA. Subsystem fs/xfs. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-80537 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| Debian 14forky | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
xfs_rtrefcountbt_compute_maxlevels() sets mp->m_rtrefc_maxlevels = min(d_maxlevels, r_maxlevels) + 1; where the trailing "+ 1" already accounts for the inode-root level, so the deepest valid on-disk root level is m_rtrefc_maxlevels - 1 and a cursor must satisfy bc_nlevels <= bc_maxlevels (= m_rtrefc_maxlevels). The two on-disk validation paths, xfs_rtrefcountbt_verify() and xfs_iformat_rtrefcount(), check the root level with ">" instead of ">=", so a crafted rtreflink (metadir + realtime + reflink) image whose /rtgroups/N.refcount inode has bb_level == m_rtrefc_maxlevels is accepted on mount. xfs_rtrefcountbt_init_cursor() then sets bc_nlevels = bb_level + 1, exceeding bc_maxlevels by one. Since the xfs_rtrefcountbt_cur slab object is sized for exactly bc_maxlevels entries, the first btree op on such a cursor indexes bc_levels[m_rtrefc_maxlevels] past the end of the object. This is reached by the first rtrefcount cursor built after mount, via log/CoW recovery (xfs_reflink_recover_cow() during xfs_mountfs()) or an FS_IOC_GETFSMAP over the realtime device. Reject a root level equal to m_rtrefc_maxlevels, matching the ">=" form already used by the sibling data-device refcount/rmap verifiers and the in-memory rtrmap verifier. BUG: KASAN: slab-out-of-bounds in xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101) Write of size 2 at addr ffff888018391658 by task exploit/144 xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101) xfs_btree_query_range (fs/xfs/libxfs/xfs_btree.c:5308) xfs_refcount_recover_cow_leftovers (fs/xfs/libxfs/xfs_refcount.c:2113) xfs_reflink_recover_cow (fs/xfs/xfs_reflink.c:1085) xlog_recover_finish (fs/xfs/xfs_log_recover.c:3551) xfs_mountfs (fs/xfs/xfs_mount.c:1158) xfs_fs_fill_super (fs/xfs/xfs_super.c:1940) get_tree_bdev_flags (fs/super.c:1634) vfs_get_tree (fs/super.c:1694) path_mount (fs/namespace.c:4161) __x64_sys_mount (fs/namespace.c:4367) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) The buggy address belongs to the cache xfs_rtrefcountbt_cur of size 216 The buggy address is located 8 bytes to the right of allocated 216-byte region [ffff888018391578, ffff888018391650) Kernel panic - not syncing: Fatal exception
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.
fs/xfs/libxfs/xfs_rtrefcount_btree.c
- What this is
- XFS FILESYSTEMMAINTAINERS section matched on fs/xfs/ · linux-xfs@vger.kernel.org · Supported
Show whether Debian builds this, per release
Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.
- bookworm, trixie, forky, sid?
- ?
- not established - no Makefile entry for the affected file
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
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
Signals
- Published
- 2026-08-26 (2 months 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 6.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- fs/xfs
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.14 | 6.18.46 | 8a0ecae2ecda |
| 6.14 | 7.1.10 | ccebfc309441 |
| 6.14 | 7.2 | cc3144da377d |
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
- The bug is reached only through local kernel entry points—mount(2) during xfs_reflink_recover_cow() in xfs_mountfs(), or FS_IOC_GETFSMAP on an already-mounted XFS volume—not via any network-facing protocol handler (NFS/ksmbd/SMB/TCP).
- AC:LAttack complexity, low
- An attacker fully controls the crafted rtreflink image and can set /rtgroups/N.refcount bb_level == m_rtrefc_maxlevels so mount or the first rtrefcount cursor reliably drives xfs_btree_lookup() to write past bc_levels[]; no victim-dependent race or layout luck is required.
- PR:LPrivileges required, low
- Triggering mount of a loop-backed crafted image needs only CAP_SYS_ADMIN inside a user namespace (unshare -Urn), which unprivileged users can obtain on typical systems; FS_IOC_GETFSMAP is additionally reachable with only a readable open file on the mounted filesystem.
- UI:NUser interaction, none
- In the highest-impact scenario an unprivileged attacker mounts their own malicious XFS image in a user namespace and immediately hits CoW recovery during xfs_mountfs(), with no separate victim action beyond routine local code execution.
- S:UScope, unchanged
- Exploitation corrupts kernel heap metadata and can panic or compromise the host kernel, but does not cross a CVSS scope boundary such as a VM/hypervisor guest-to-host escape or IOMMU/DMA sandbox breakout.
- C:HConfidentiality impact, high
- KASAN reports a slab out-of-bounds write in xfs_btree_lookup() past the xfs_rtrefcountbt_cur allocation; such heap corruption can disclose or influence adjacent slab objects and is treated as arbitrary memory-read potential under kernel CVSS guidance.
- I:HIntegrity impact, high
- The vulnerability is an out-of-bounds write (2 bytes into bc_levels[m_rtrefc_maxlevels]) on a kmalloc slab cursor object, enabling adjacent-object corruption and further heap shaping toward arbitrary kernel write or code execution.
- A:HAvailability impact, high
- The reported reproducer causes KASAN slab-out-of-bounds followed by kernel panic during mount recovery; any host mounting or querying the malicious rtrefcount metadata can crash or deny service to the entire system.