CVE-2021-3491: Canonical Ubuntu Linux

High severity, CVSS 8.8. EPSS: 0.6% chance of exploitation in the next 30 days.

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).

Affected products

• Canonical Ubuntu Linux: version 20.04 only; version 20.10 only; version 21.04 only
• Linux Linux Kernel: from 5.7, before 5.10.37 (fixed in 5.10.37); from 5.11, before 5.11.21 (fixed in 5.11.21); from 5.12, before 5.12.4 (fixed in 5.12.4)

Published 2021-06-04. Last modified 2026-06-17.