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CVE-2025-37777
ksmbd: fix use-after-free in __smb2_lease_break_noti()
Published 2025-05-01 (18 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-2025-37777 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 | fixed | 6.12.27-1 | Update the kernel and reboot. Fixed in 6.12.27-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 6.12.27-1 | Update the kernel and reboot. Fixed in 6.12.27-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.12.27-1 | Update the kernel and reboot. Fixed in 6.12.27-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Move tcp_transport free to ksmbd_conn_free. If ksmbd connection is referenced when ksmbd server thread terminates, It will not be freed, but conn->tcp_transport is freed. __smb2_lease_break_noti can be performed asynchronously when the connection is disconnected. __smb2_lease_break_noti calls ksmbd_conn_write, which can cause use-after-free when conn->ksmbd_transport is already freed.
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/server/connection.c
fs/smb/server/transport_tcp.c
fs/smb/server/transport_tcp.h
- What this is
- KERNEL SMB3 SERVER (KSMBD)MAINTAINERS section matched on fs/smb/server/ · linux-cifs@vger.kernel.org · Maintained
- Kernel module
- ksmbd
Check it on the machine
grep -wE 'CONFIG_SMBFS|CONFIG_SMB_SERVER' /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 ksmbdPrints 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_SMBFS, CONFIG_SMB_SERVER
- 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
- 2025-05-01 (18 months ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.37%, higher than 29.5% 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 |
|---|---|---|
| 5.15 | 6.6.101 | 1da8bd9a10ec |
| 5.15 | 6.12.26 | 1aec4d14cf81 |
| 5.15 | 6.14.4 | e59796fc8060 |
| 5.15 | 6.15 | 21a4e47578d4 |
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:NAttack vector, network
- ksmbd is the in-kernel SMB server listening on TCP/445, and the entire trigger sequence (CREATE with a lease context, TCP disconnect, conflicting CREATE from a second connection) is driven purely by remote SMB2 traffic. No local access is needed.
- AC:LAttack complexity, low
- The attacker fully controls both sides of the race by opening two connections — one takes a lease and disconnects, the other forces the lease break — and can repeat it indefinitely until the RCU-deferred opinfo teardown overlaps the transport free. Per guidance, an attacker-created race and a use-after-free are Low.
- PR:LPrivileges required, low
- Reaching `alloc_opinfo()` requires a completed SESSION_SETUP, TREE_CONNECT and SMB2 CREATE with an RqLs lease context, so an authenticated (any unprivileged share user, including guest where mapped) SMB account is needed. This matches the scoring of the sibling post-authentication ksmbd UAFs.
- UI:NUser interaction, none
- The attacker drives every step from its own SMB connections; no action by an administrator or any other client is required.
- S:UScope, unchanged
- The corruption and its consequences are confined to the kernel's own security authority, with no crossing of a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- The freed kmalloc-64 `tcp_transport` can be reclaimed with attacker-controlled data and its `ops` field is dereferenced, which combined with the indirect call yields arbitrary kernel memory disclosure; the already-released socket is also reused.
- I:HIntegrity impact, high
- `ksmbd_conn_write()` reads the `ops` vtable pointer out of the freed object and indirectly calls `ops->writev`, giving a direct control-flow hijack primitive and hence arbitrary kernel code execution after heap spraying.
- A:HAvailability impact, high
- Even unweaponized, dereferencing the freed transport and the released socket reliably oopses or panics the kernel, and the attacker can trigger it repeatedly to keep the server down.