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CVE-2026-72315
smb: client: fix busy dentry warning on unmount after DIO
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72315 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | fixedDSA-6568-1 · 2026-10-16 · text | 6.12.113-1 | Update the kernel and reboot. DSA-6568-1 shipped the fix on 2026-10-16, in 6.12.113-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Commit c68337442f03 ("cifs: Fix busy dentry used after unmounting") fixed the issue in cifs where deferred close of a file led to a dentry reference count not being released in umount, by flushing deferredclose_wq in cifs_kill_sb() to solve it. However, the cifs DIO path suffers from the same busy-dentry problem caused by a delayed dentry reference-count release: [dio] [cifsd] [close + umount] netfs_unbuffered_write_iter_locked ... cifs_demultiplex_thread netfs_unbuffered_write cifs_issue_write netfs_wait_for_in_progress_stream [1] ... netfs_write_subrequest_terminated netfs_subreq_clear_in_progress netfs_wake_collector // wake [1] netfs_put_subrequest netfs_put_request queue_work(system_dfl_wq, xxx) [2] // dio write return cifs_close _cifsFileInfo_put // cfile->count 2->1 --cfile->count [3] // umount cifs_kill_sb kill_anon_super // warning triggered! shrink_dcache_for_umount [4] [system_dfl_wq] [5] netfs_free_request ... _cifsFileInfo_put // cfile->count 1->0 --cfile->count queue_work(fileinfo_put_wq, xxx) [fileinfo_put_wq] [6] cifsFileInfo_put_work cifsFileInfo_put_final dput If the umount path is triggered before [5], it results warning: BUG: Dentry 00000000eab1f070{i=9a917b66ae404fec,n=test} still in use (1) [unmount of cifs cifs] The existing per-inode ictx->io_count wait in cifs_evict_inode() does not help: it lives in the inode eviction path, which runs after shrink_dcache_for_umount() has already warned about the busy dentries. Fix it by adding a per-superblock outstanding-rreq counter that is incremented in cifs_init_request() and decremented in cifs_free_request(). In cifs_kill_sb(), before kill_anon_super(), wait for this counter to reach 0 - which guarantees that all cleanup_work for this sb have run and thus all relevant cfile puts are queued on fileinfo_put_wq or serverclose_wq. Then drain the workqueue so the dentry refs are dropped. This is a targeted wait, not a flush of the system-wide system_dfl_wq.
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/smb/client/cifs_fs_sb.h
fs/smb/client/cifsfs.c
fs/smb/client/connect.c
fs/smb/client/file.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /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 cifsPrints 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, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_CIFS, CONFIG_SMBFS
- 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
- 2026-08-15 (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
- 0.18%, higher than 6.5% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/smb
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.12.78 | 6.12.112 | f7748b551010 |
| 6.18.20 | 6.18.54 | 9fc87899276a |
| 7.0 | 7.1.5 | f0eac9c3c371 |
| 7.0 | 7.2 | 75f5c412fa86 |
| 6.1.167 | 0 | 0 |
| 6.6.130 | 0 | 0 |
| 6.19.10 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DSA-6568-1 | 2026-10-16 | trixie 6.12.113-1 | 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 flaw is reached through local VFS syscalls (open with O_DIRECT, pwrite, close, umount) on the in-kernel CIFS/SMB3 client mount teardown path, not via remote SMB packet handling or ksmbd server code.
- AC:LAttack complexity, low
- The attacker controls both sides of the race by issuing concurrent O_DIRECT writes and close/umount from separate threads, can flood system_dfl_wq to widen the cleanup delay window, and can retry until the post-pwrite/pre-cleanup_work window is hit.
- PR:LPrivileges required, low
- Exploitation requires only a local user with write access to files on an already-mounted CIFS/SMB3 share; mount/umount can be performed by the same attacker with namespace capabilities or triggered by autofs lazy-unmount on shared enterprise home-directory mounts.
- UI:NUser interaction, none
- No victim interaction is required; the attacker directly drives the open/pwrite/close/umount syscall sequence in a tight loop without relying on another user to open files or perform any action.
- S:UScope, unchanged
- Impact is confined to kernel heap corruption and privilege escalation within the same host; this is not a VM escape, sandbox escape, or cross-security-authority boundary flaw.
- C:HConfidentiality impact, high
- Delayed netfs cleanup causes cifsFileInfo_put_final() to dereference dentry, inode, and cifsInodeInfo structures after superblock teardown has begun, constituting a use-after-free that can be leveraged for arbitrary kernel memory read.
- I:HIntegrity impact, high
- Post-unmount cifsFileInfo_put_final() performs lock operations and list manipulation on freed kernel objects via fileinfo_put_wq, enabling heap metadata corruption and control-flow hijacking for arbitrary write or code execution.
- A:HAvailability impact, high
- The race triggers WARN in shrink_dcache_for_umount during umount, and subsequent use-after-free in the deferred dentry put worker can cause kernel oops, panic, or hang.