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CVE-2025-38734
net/smc: fix UAF on smcsk after smc_listen_out()
Published 2025-09-05 (13 months ago) by the Linux kernel CNA. Subsystem net/smc. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2025-38734 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-6008-1 · 2025-09-22 · text | 6.12.48-1 | Update the kernel and reboot. DSA-6008-1 shipped the fix on 2025-09-22, in 6.12.48-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 6.16.5-1 | Update the kernel and reboot. Fixed in 6.16.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.16.5-1 | Update the kernel and reboot. Fixed in 6.16.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
BPF CI testing report a UAF issue: [ 16.446633] BUG: kernel NULL pointer dereference, address: 000000000000003 0 [ 16.447134] #PF: supervisor read access in kernel mod e [ 16.447516] #PF: error_code(0x0000) - not-present pag e [ 16.447878] PGD 0 P4D 0 [ 16.448063] Oops: Oops: 0000 [#1] PREEMPT SMP NOPT I [ 16.448409] CPU: 0 UID: 0 PID: 9 Comm: kworker/0:1 Tainted: G OE 6.13.0-rc3-g89e8a75fda73-dirty #4 2 [ 16.449124] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODUL E [ 16.449502] Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/201 4 [ 16.450201] Workqueue: smc_hs_wq smc_listen_wor k [ 16.450531] RIP: 0010:smc_listen_work+0xc02/0x159 0 [ 16.452158] RSP: 0018:ffffb5ab40053d98 EFLAGS: 0001024 6 [ 16.452526] RAX: 0000000000000001 RBX: 0000000000000002 RCX: 000000000000030 0 [ 16.452994] RDX: 0000000000000280 RSI: 00003513840053f0 RDI: 000000000000000 0 [ 16.453492] RBP: ffffa097808e3800 R08: ffffa09782dba1e0 R09: 000000000000000 5 [ 16.453987] R10: 0000000000000000 R11: 0000000000000000 R12: ffffa0978274640 0 [ 16.454497] R13: 0000000000000000 R14: 0000000000000000 R15: ffffa09782d4092 0 [ 16.454996] FS: 0000000000000000(0000) GS:ffffa097bbc00000(0000) knlGS:000000000000000 0 [ 16.455557] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003 3 [ 16.455961] CR2: 0000000000000030 CR3: 0000000102788004 CR4: 0000000000770ef 0 [ 16.456459] PKRU: 5555555 4 [ 16.456654] Call Trace : [ 16.456832] <TASK > [ 16.456989] ? __die+0x23/0x7 0 [ 16.457215] ? page_fault_oops+0x180/0x4c 0 [ 16.457508] ? __lock_acquire+0x3e6/0x249 0 [ 16.457801] ? exc_page_fault+0x68/0x20 0 [ 16.458080] ? asm_exc_page_fault+0x26/0x3 0 [ 16.458389] ? smc_listen_work+0xc02/0x159 0 [ 16.458689] ? smc_listen_work+0xc02/0x159 0 [ 16.458987] ? lock_is_held_type+0x8f/0x10 0 [ 16.459284] process_one_work+0x1ea/0x6d 0 [ 16.459570] worker_thread+0x1c3/0x38 0 [ 16.459839] ? __pfx_worker_thread+0x10/0x1 0 [ 16.460144] kthread+0xe0/0x11 0 [ 16.460372] ? __pfx_kthread+0x10/0x1 0 [ 16.460640] ret_from_fork+0x31/0x5 0 [ 16.460896] ? __pfx_kthread+0x10/0x1 0 [ 16.461166] ret_from_fork_asm+0x1a/0x3 0 [ 16.461453] </TASK > [ 16.461616] Modules linked in: bpf_testmod(OE) [last unloaded: bpf_testmod(OE) ] [ 16.462134] CR2: 000000000000003 0 [ 16.462380] ---[ end trace 0000000000000000 ]--- [ 16.462710] RIP: 0010:smc_listen_work+0xc02/0x1590 The direct cause of this issue is that after smc_listen_out_connected(), newclcsock->sk may be NULL since it will releases the smcsk. Therefore, if the application closes the socket immediately after accept, newclcsock->sk can be NULL. A possible execution order could be as follows: smc_listen_work | userspace ----------------------------------------------------------------- lock_sock(sk) | smc_listen_out_connected() | | \- smc_listen_out | | | \- release_sock | | |- sk->sk_data_ready() | | fd = accept(); | close(fd); | \- socket->sk = NULL; /* newclcsock->sk is NULL now */ SMC_STAT_SERV_SUCC_INC(sock_net(newclcsock->sk)) Since smc_listen_out_connected() will not fail, simply swapping the order of the code can easily fix this issue.
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.
net/smc/af_smc.c
- What this is
- SHARED MEMORY COMMUNICATIONS (SMC) SOCKETSMAINTAINERS section matched on net/smc/ · linux-rdma@vger.kernel.org · Supported
- Kernel module
- smc
Check it on the machine
grep -w CONFIG_SMC /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 smcPrints 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 does not build this code in 2 releases. It does build it in 2 releases, so the answer depends on which release the machine runs.
- bookworm, trixiem
- forky, sidn
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- n
- not enabled on any flavour - this release does not build the code
- Build switch
- CONFIG_SMC
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
- 2025-09-05 (13 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.46%, higher than 38.9% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/smc
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.18 | 6.6.103 | 070b4af44c4b |
| 4.18 | 6.12.44 | 2e765ba0ee0e |
| 4.18 | 6.16.4 | 85545f1525f9 |
| 4.18 | 6.17 | d9cef55ed491 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DSA-6008-1 | 2025-09-22 | trixie 6.12.48-1 | 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:NAttack vector, network
- The vulnerable `smc_listen_work()` handler is scheduled and driven entirely by an inbound TCP connection carrying the SMC experimental option and the subsequent CLC handshake messages from a remote peer, making this a net/ stack bug reachable via received packets.
- AC:LAttack complexity, low
- The attacker can open unlimited concurrent SMC connections and retry the race indefinitely, and in the loopback-ism/local-peer scenario controls both sides outright (the connect and the accept()+close()); the race was reproduced reliably enough to be caught by CI.
- PR:NPrivileges required, none
- The SMC CLC handshake performs no authentication whatsoever — the connection is accepted and the vulnerable worker runs before any credential check, so an unauthenticated remote peer reaches the code.
- UI:NUser interaction, none
- No deliberate victim action is required; the server application's normal accept()/close() sequence is what races the worker, and the attacker triggers the handshake itself.
- S:UScope, unchanged
- The use-after-free and resulting corruption are confined to kernel memory within the same security authority; no VM, IOMMU, or sandbox boundary is crossed.
- C:HConfidentiality impact, high
- This is a use-after-free read of a freed `struct socket`, and the stale value is dereferenced as a `struct sock *` and chased through `sk_net` — with the freed slab reclaimed under attacker influence this yields arbitrary kernel-memory reads.
- I:HIntegrity impact, high
- The pointer read from the freed object is used as the base for `this_cpu_inc()` in SMC_STAT_SERV_SUCC_INC, turning the UAF into an attacker-influenceable kernel write primitive suitable for heap grooming and control-flow hijacking.
- A:HAvailability impact, high
- The bug reproducibly oopses the smc_hs_wq kworker with a supervisor page fault (confirmed in the commit's trace), and a remote attacker can trigger it repeatedly to panic or destabilize the system.