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Fencer Vulns
high 7.8

CVE-2025-38463 json

tcp: Correct signedness in skb remaining space calculation

published 2025-07-25 15 mo ago modified 2026-08-05 3 mo ago also EUVD-2025-22678

cvss
7.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 8%
public exploits
0
none known

Summary

In the Linux kernel, the following vulnerability has been resolved:

tcp: Correct signedness in skb remaining space calculation

Syzkaller reported a bug [1] where sk->sk_forward_alloc can overflow.

When we send data, if an skb exists at the tail of the write queue, the kernel will attempt to append the new data to that skb. However, the code that checks for available space in the skb is flawed: ''' copy = size_goal - skb->len '''

The types of the variables involved are: ''' copy: ssize_t (s64 on 64-bit systems) size_goal: int skb->len: unsigned int '''

Due to C's type promotion rules, the signed size_goal is converted to an unsigned int to match skb->len before the subtraction. The result is an unsigned int.

When this unsigned int result is then assigned to the s64 copy variable, it is zero-extended, preserving its non-negative value. Consequently, copy is always >= 0.

Assume we are sending 2GB of data and size_goal has been adjusted to a value smaller than skb->len. The subtraction will result in copy holding a very large positive integer. In the subsequent logic, this large value is used to update sk->sk_forward_alloc, which can easily cause it to overflow.

The syzkaller reproducer uses TCP_REPAIR to reliably create this condition. However, this can also occur in real-world scenarios. The tcp_bound_to_half_wnd() function can also reduce size_goal to a small value. This would cause the subsequent tcp_wmem_schedule() to set sk->sk_forward_alloc to a value close to INT_MAX. Further memory allocation requests would then cause sk_forward_alloc to wrap around and become negative.

[1]: https://syzkaller.appspot.com/bug?extid=de6565462ab540f50e47

Patch / fix

  • Linux Linux 6.6.99, 6.12.39, 6.15.7, 6.16
  • linux Debian 13 6.12.41-1
  • linux Debian 14 6.16.3-1

debian advisories DEBIAN-CVE-2025-38463

  • linux Ubuntu 24.04 LTS 6.8.0-100.100
  • linux-aws Ubuntu 24.04 LTS 6.8.0-1046.49
  • linux-gcp Ubuntu 24.04 LTS 6.8.0-1047.50
  • linux-gke Ubuntu 24.04 LTS 6.8.0-1043.48
  • linux-ibm Ubuntu 24.04 LTS 6.8.0-1044.44
  • linux-azure Ubuntu 24.04 LTS 6.8.0-1046.52
  • linux-gkeop Ubuntu 24.04 LTS 6.8.0-1030.33
  • linux-raspi Ubuntu 24.04 LTS 6.8.0-1047.51

ubuntu advisories UBUNTU-CVE-2025-38463

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel RHEL 10.0 6.12.0-55.33.1.el10_0
  • rv RHEL 10.0 6.12.0-55.33.1.el10_0
  • perf RHEL 10.0 6.12.0-55.33.1.el10_0
  • rtla RHEL 10.0 6.12.0-55.33.1.el10_0
  • libperf RHEL 10.0 6.12.0-55.33.1.el10_0
  • kernel-rt RHEL 10.0 6.12.0-55.33.1.el10_0
  • kernel-64k RHEL 10.0 6.12.0-55.33.1.el10_0
  • kernel-core RHEL 10.0 6.12.0-55.33.1.el10_0

red hat advisories RHSA-2025:15782

  • kernel-rt Linux Micro 6.2 6.12.0-160000.6.1
  • kernel-64kb SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-source SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-default SLES 16.0, SLE High Availability Extension 16.0, Linux Micro 6.2 +1 more 6.12.0-160000.6.1
  • kernel-default-base SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1.160000.2.4
  • kernel-syms SLES 16.0, Linux Micro Extras 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-obs-qa SLES for SAP applications 16.0, SLES 16.0 6.12.0-160000.6.1
  • kernel-kvmsmall SLES for SAP applications 16.0, SLES 16.0 6.12.0-160000.6.1

suse advisories SUSE-SU-2025:21074-1SUSE-SU-2025:21139-1SUSE-SU-2026:20560-1SUSE-SU-2025:21179-1SUSE-SU-2025:03023-1SUSE-SU-2025:03011-1

  • kernel-rt openSUSE Leap 16.0 6.12.0-160000.6.1
  • dtb-aarch64 openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-64kb openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-syms openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-obs-qa openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-source openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-default openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-kvmsmall openSUSE Leap 16.0 6.12.0-160000.6.1

opensuse advisories openSUSE-SU-2025:20081-1

  • kernel-abi-stablelists AlmaLinux 10 6.12.0-55.33.1.el10_0

almalinux advisories ALSA-2025:15782

  • kernel Azure Linux 3.0 6.6.104.2-1

azure linux advisories AZL-65774

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2025-38463 updated cvss 7.3

kernel: tcp: Correct signedness in skb remaining space calculation

  • Red Hat Enterprise Linux AppStream (v. 10) rv 6.12.0-55.33.1.el10_0, perf 6.12.0-55.33.1.el10_0, rtla 6.12.0-55.33.1.el10_0, python3-perf 6.12.0-55.33.1.el10_0, kernel-devel-matched 6.12.0-55.33.1.el10_0, kernel-64k-devel-matched 6.12.0-55.33.1.el10_0, +7 more Red Hat Enterprise Linux BaseOS (v. 10) kernel 6.12.0-55.33.1.el10_0, kernel-64k 6.12.0-55.33.1.el10_0, kernel-core 6.12.0-55.33.1.el10_0, kernel-tools 6.12.0-55.33.1.el10_0, kernel-modules 6.12.0-55.33.1.el10_0, kernel-64k-core 6.12.0-55.33.1.el10_0, +27 more Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) libperf 6.12.0-55.33.1.el10_0, kernel-debuginfo-common-s390x 6.12.0-55.33.1.el10_0, kernel-debuginfo-common-x86_64 6.12.0-55.33.1.el10_0, kernel-debuginfo-common-aarch64 6.12.0-55.33.1.el10_0, kernel-debuginfo-common-ppc64le 6.12.0-55.33.1.el10_0 Red Hat Enterprise Linux Real Time (v. 10) kernel-rt 6.12.0-55.33.1.el10_0, kernel-rt-64k 6.12.0-55.33.1.el10_0, kernel-rt-core 6.12.0-55.33.1.el10_0, kernel-rt-modules 6.12.0-55.33.1.el10_0, kernel-rt-64k-core 6.12.0-55.33.1.el10_0, kernel-rt-debug-core 6.12.0-55.33.1.el10_0, +14 more Red Hat Enterprise Linux Real Time for NFV (v. 10) kernel-rt 6.12.0-55.33.1.el10_0, kernel-rt-kvm 6.12.0-55.33.1.el10_0, kernel-rt-core 6.12.0-55.33.1.el10_0, kernel-rt-modules 6.12.0-55.33.1.el10_0, kernel-rt-debug-kvm 6.12.0-55.33.1.el10_0, kernel-rt-debug-core 6.12.0-55.33.1.el10_0, +6 more
    fixed: vendor fix RHSA-2025:15782
  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 8% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • Linux Linux

Classification

Weakness
  • CWE-191 Integer Underflow (Wrap or Wraparound)
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.