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CVE-2026-63970
vsock/virtio: bind uarg before filling zerocopy skb
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/vmw_vsock. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-63970 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 | fixedDSA-6405-1 · 2026-07-31 · text | 6.12.100-1 | Update the kernel and reboot. DSA-6405-1 shipped the fix on 2026-07-31, in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4724-1 · 2026-08-07 · text | 6.12.100-1~deb12u1 | Update the kernel and reboot. DLA-4724-1 shipped the fix on 2026-08-07, in 6.12.100-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
virtio_transport_send_pkt_info() allocates or reuses the zerocopy uarg before entering the send loop, but virtio_transport_alloc_skb() still fills the skb before it inherits that uarg. When fixed-buffer vectored zerocopy hits MAX_SKB_FRAGS, io_sg_from_iter() may partially attach managed frags and return -EMSGSIZE. The rollback path call kfree_skb() to free an skb that carries SKBFL_MANAGED_FRAG_REFS but no uarg, so skb_release_data() falls through to ordinary frag unref. Pass the uarg into virtio_transport_alloc_skb() and bind it immediately before virtio_transport_fill_skb(). This keeps control or no-payload skbs untouched while ensuring success and rollback share one lifetime rule.
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/vmw_vsock/virtio_transport_common.c
- What this is
- VIRTIO AND VHOST VSOCK DRIVERMAINTAINERS section matched on net/vmw_vsock/virtio_transport_common.c · kvm@vger.kernel.org · Maintained
- Kernel module
- vmw_vsock_virtio_transport_common
Check it on the machine
grep -wE 'CONFIG_VIRTIO_VSOCKETS_COMMON|CONFIG_VSOCKETS' /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 vmw_vsock_virtio_transport_commonPrints 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 switches
- CONFIG_VIRTIO_VSOCKETS_COMMON, CONFIG_VSOCKETS
- 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
- 2026-07-19 (3 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 6.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- net/vmw_vsock
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.7 | 6.12.97 | 72194f650509 |
| 6.7 | 6.18.35 | b62e2b2b4a50 |
| 6.7 | 7.0.12 | 5d317573f1d4 |
| 6.7 | 7.1 | 1e584c304cfb |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4724-1 | 2026-08-07 | bookworm 6.12.100-1~deb12u1 | tracker |
| DSA-6405-1 | 2026-07-31 | trixie 6.12.100-1 | 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 bug is reached only via local AF_VSOCK transmission (syscall/io_uring send on a connected virtio vsock socket), not by processing remote network packets. Per kernel CVSS guidance, vsock is Local.
- AC:LAttack complexity, low
- An attacker fully controls the trigger: SO_ZEROCOPY, io_uring IORING_OP_SEND_ZC with IORING_RECVSEND_FIXED_BUF and a vectored iov exceeding MAX_SKB_FRAGS (17), causing io_sg_from_iter() to return -EMSGSIZE on a deterministic error rollback path.
- PR:LPrivileges required, low
- Exploitation requires only an unprivileged local process that can open a vsock connection and use io_uring zerocopy send; no CAP_NET_ADMIN or real root is needed beyond ordinary local user access in a virtio-vsock environment (KVM/Firecracker/cloud microVMs).
- UI:NUser interaction, none
- No victim interaction is required; the attacker triggers the bug directly through their own socket send/io_uring operation.
- S:UScope, unchanged
- Impact is kernel memory corruption and privilege escalation within the same security domain where the send occurs (guest kernel or host kernel). It is not a cross-boundary VM escape on the receive path; the flaw is in the sender's local skb teardown.
- C:HConfidentiality impact, high
- Incorrect __skb_frag_unref() on SKBFL_MANAGED_FRAG_REFS pages without a bound uarg corrupts refcount/lifetime of io_uring registered buffer pages, yielding use-after-free and arbitrary kernel memory read primitives.
- I:HIntegrity impact, high
- The refcount corruption is a memory-safety bug in kernel heap/page management that can be leveraged for arbitrary write and control-flow hijacking, not merely a bounded data change.
- A:HAvailability impact, high
- The erroneous frag unref on managed zerocopy pages can cause immediate kernel oops/panic from refcount underflow or use-after-free during skb teardown, independent of full exploitation.