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CVE-2026-93810
cachefiles: Fix double fput
Published 2026-09-24 (24 days ago) by the Linux kernel CNA. Subsystem fs/cachefiles. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-93810 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
Fix a double fput() in error handling in cachefiles_create_tmpfile().
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/cachefiles/namei.c
- What this is
- CACHEFILES: FS-CACHE BACKEND FOR CACHING ON MOUNTED FILESYSTEMSMAINTAINERS section matched on fs/cachefiles/ · netfs@lists.linux.dev · Supported
- Kernel module
- cachefiles
Check it on the machine
grep -w CONFIG_CACHEFILES /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 cachefilesPrints 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 switch
- CONFIG_CACHEFILES
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
- 2026-09-24 (24 days ago)source
- CVSS
- 7.0 high
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.12%, higher than 1.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/cachefiles
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.1 | 6.1.189 | e582a3e99584 |
| 6.1 | 6.6.158 | 246098a2b8e8 |
| 6.1 | 6.12.111 | e9c2e68c3890 |
| 6.1 | 6.18.53 | 90b306eee037 |
| 6.1 | 7.2 | af6830cc12df |
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
- The extra fput is in cachefiles_create_tmpfile(), reached from nfs_fscache_open_file() (or CIFS/9p/AFS/Ceph fscache_use_cookie()) → fscache_perform_lookup() → cachefiles_lookup_cookie() → cachefiles_look_up_object() → cachefiles_create_file(), or from cachefiles_invalidate_cookie(); a local open/inval of an fscache-backed file is the entry, not a remote protocol message.
- AC:HAttack complexity, high
- The double fput runs only when kernel_tmpfile_open()'s file has a NULL f_op->read_iter or f_op->write_iter. cachefiles_add_cache() requires tmpfile/xattr/sync_fs and s_blocksize<=PAGE_SIZE but never checks those f_ops; the only in-tree tmpfile FS lacking write_iter is hugetlbfs, which add_cache rejects, so an attacker cannot make a bound ext4/xfs/btrfs/f2fs tmpfile take this branch.
- PR:LPrivileges required, low
- nfs_fscache_open_file() and the sibling fscache_use_cookie() open paths have no capable() check, so an unprivileged user with read access on an already-bound cachefiles/fscache mount drives cookie lookup into cachefiles_create_tmpfile(); cachefiles_daemon_open() needs init-namespace CAP_SYS_ADMIN, but that is not this trigger.
- UI:NUser interaction, none
- The attacker themselves open or invalidate files on the fscache-enabled netfs mount, which queues fscache_perform_lookup() into cachefiles_create_tmpfile(); no other user must mount the volume or issue cachefilesd commands.
- S:UScope, unchanged
- Double-putting the cache tmpfile's struct file corrupts host kernel file-refcount and filp_cachep state only; it does not cross a KVM/Xen, IOMMU, or other distinct authority.
- C:HConfidentiality impact, high
- On the !read_iter/!write_iter branch, cachefiles_create_tmpfile() fput()s then err_unuse calls file_inode(file) and fput()s again. fscache_perform_lookup() runs in a PF_KTHREAD worker so the first put is deferred via delayed_fput_work; a later file_inode() on that pointer is a UAF read of a SLAB_TYPESAFE_BY_RCU struct file.
- I:HIntegrity impact, high
- The leftover fput before goto err_unuse is a struct-file double-put: if delayed __fput() has already file_free()'d the tmpfile, the second file_ref_put() writes a recycled filp_cachep object (or an underflowed live ref into the dead zone), a UAF write primitive against struct file.
- A:HAvailability impact, high
- The second put hits __file_ref_put_badval()'s "imbalanced put on file reference count" WARN_ONCE, and if delayed_fput already ran, file_inode()/inode_lock/fput in cachefiles_create_tmpfile() err_unuse dereference freed memory and oops the fscache worker.