CVE-2025-21820: linux kernel — Fix

Description

In the Linux kernel, the following vulnerability has been resolved: tty: xilinx_uartps: split sysrq handling lockdep detects the following circular locking dependency: CPU 0 CPU 1 ========================== ============================ cdns_uart_isr() printk() uart_port_lock(port) console_lock() cdns_uart_console_write() if (!port->sysrq) uart_port_lock(port) uart_handle_break() port->sysrq = ... uart_handle_sysrq_char() printk() console_lock() The fixed commit attempts to avoid this situation by only taking the port lock in cdns_uart_console_write if port->sysrq unset. However, if (as shown above) cdns_uart_console_write runs before port->sysrq is set, then it will try to take the port lock anyway. This may result in a deadlock. Fix this by splitting sysrq handling into two parts. We use the prepare helper under the port lock and defer handling until we release the lock.

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CVSS v3.1 Score

5.5

MEDIUM

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Weakness Type (CWE)

CWE-667 CWE-667

Affected Products

Vendor	Product	Versions
linux	linux_kernel	4.6 — 6.1.129
linux	linux_kernel	6.2 — 6.6.78
linux	linux_kernel	6.7 — 6.12.14
linux	linux_kernel	6.13 — 6.13.3

References

Advisories & Patches

https://git.kernel.org/stable/c/4410dba9807a17a93f649a9f5870ceaf30a675a3 https://git.kernel.org/stable/c/8ea0e7b3d7b8f2f0fc9db491ff22a0abe120801c https://git.kernel.org/stable/c/9b88a7c4584ba67267a051069b8abe44fc9595b2 https://git.kernel.org/stable/c/b06f388994500297bb91be60ffaf6825ecfd2afe https://git.kernel.org/stable/c/de5bd24197bd9ee37ec1e379a3d882bbd15c5065

Other References

https://git.kernel.org/stable/c/e22a97700901ba5e8bf8db68056a0d50f9440cae https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html

Frequently Asked Questions

What is CVE-2025-21820? +

In the Linux kernel, the following vulnerability has been resolved: tty: xilinx_uartps: split sysrq handling lockdep detects the following circular locking dependency: CPU 0 CPU 1 ========================== ============================ cdns_uart_isr() printk() uart_port_lock(port) console_lock() cdns_uart_console_write() if (!port->sysrq) uart_port_lock(port) uart_handle_break() port->sysrq = ... uart_handle_sysrq_char() printk() console_lock() The fixed commit attempts to avoid this situation by only taking the port lock in cdns_uart_console_write if port->sysrq unset. However, if (as shown above) cdns_uart_console_write runs before port->sysrq is set, then it will try to take the port lock anyway. This may result in a deadlock. Fix this by splitting sysrq handling into two parts. We use the prepare helper under the port lock and defer handling until we release the lock. It has a CVSS v3.1 base score of 5.5 (MEDIUM).

How severe is CVE-2025-21820? +

CVE-2025-21820 has a CVSS v3.1 score of 5.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-21820? +

CVE-2025-21820 affects products from linux, specifically: linux_kernel. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-21820? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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