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Fencer Vulns
high 7.8

CVE-2021-3491 json

Linux kernel io_uring PROVIDE_BUFFERS MAX_RW_COUNT bypass

published 2021-06-04 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-26813

cvss
7.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.6%
EPSS 0.6% chance of exploitation in 30 days, percentile 51%
public exploits
0
none known

Summary

The io_uring subsystem in the Linux kernel allowed the MAX_RW_COUNT limit to be bypassed in the PROVIDE_BUFFERS operation, which led to negative values being usedin mem_rw when reading /proc/<PID>/mem. This could be used to create a heap overflow leading to arbitrary code execution in the kernel. It was addressed via commit d1f82808877b ("io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers") (v5.13-rc1) and backported to the stable kernels in v5.12.4, v5.11.21, and v5.10.37. It was introduced in ddf0322db79c ("io_uring: add IORING_OP_PROVIDE_BUFFERS") (v5.7-rc1).

Patch / fix

  • Linux Linux kernel v5.10.37, v5.11.21, v5.12.4, v5.13-rc4
  • linux Debian 14, Debian 13, Debian 12 5.10.38-1

debian advisories DEBIAN-CVE-2021-3491

  • linux-hwe-5.8 Ubuntu 20.04 LTS 5.8.0-53.60~20.04.1
  • linux-oem-5.10 Ubuntu 20.04 LTS 5.10.0-1026.27
  • linux-riscv-5.8 Ubuntu 20.04 LTS 5.8.0-25.27~20.04.1

ubuntu advisories UBUNTU-CVE-2021-3491

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-syms SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 4.12.14-150.75.1
  • kernel-source SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 4.12.14-150.75.1
  • kernel-default SLES for SAP 15, SLES 15-LTSS, SLE Live Patching 15 +3 more 4.12.14-150.75.1
  • kernel-vanilla SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 4.12.14-150.75.1
  • kernel-obs-build SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 4.12.14-150.75.1
  • kernel-rt SLE Micro 5.0, Real Time Module 15 SP3 5.3.18-8.13.1
  • kernel-syms-rt Real Time Module 15 SP3 5.3.18-8.13.1
  • kernel-rt_debug Real Time Module 15 SP3 5.3.18-8.13.1

suse advisories SUSE-SU-2021:2421-1SUSE-SU-2021:2208-1SUSE-SU-2021:1977-1SUSE-SU-2021:1975-1SUSE-SU-2021:1913-1SUSE-SU-2021:1912-1

  • kernel-source openSUSE Tumbleweed 5.14.6-1.4
  • kernel-source openSUSE Tumbleweed 6.6.17-1.1
  • kernel-64kb openSUSE Leap 15.3 5.3.18-59.5.2
  • kernel-syms openSUSE Leap 15.3 5.3.18-59.5.1
  • kernel-obs-qa openSUSE Leap 15.3 5.3.18-59.5.1
  • kernel-source openSUSE Leap 15.3 5.3.18-59.5.2
  • kernel-default openSUSE Leap 15.3 5.3.18-59.5.2
  • kernel-preempt openSUSE Leap 15.3 5.3.18-59.5.2

opensuse advisories openSUSE-SU-2024:10728-1openSUSE-SU-2024:13704-1openSUSE-SU-2021:1977-1openSUSE-SU-2021:1975-1openSUSE-SU-2021:0947-1openSUSE-SU-2021:0843-1

  • linux Arch Linux 5.12.4.arch1-1
  • linux-zen Arch Linux 5.12.4.zen1-1
  • linux-hardened Arch Linux 5.11.21.hardened1-1
  • linux-lts Arch Linux 5.10.37-1

arch linux advisories AVG-1958AVG-1959AVG-1960AVG-1961

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-2021-3491 updated cvss 7.4

kernel: Linux kernel io_uring PROVIDE_BUFFERS MAX_RW_COUNT bypass

  • 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, +82 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

0.64% chance of exploitation in the next 30 days, higher than 51% 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:H/PR:L/UI:N/S:C/C:H/I:H/A:H
NVD 3.1 8.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
NVD 2.0 7.2 AV:L/AC:L/Au:N/C:C/I:C/A:C

Affected products

  • Linux Linux kernel

Classification

Weakness
  • CWE-131 Incorrect Calculation of Buffer Size
  • CWE-787 Out-of-bounds Write
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.