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CVE Record

CVE-2025-21820: tty: xilinx_uartps: split sysrq handling

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2025-21820 is a Linux kernel deadlock bug in the Xilinx UARTPS serial driver. Under specific console and SysRq handling timing, the kernel can wait on locks in a circular pattern, affecting availability on systems using this driver.

Executive priority

Treat as a targeted availability risk for affected embedded Linux fleets, not a broad emergency. Prioritize patching where these devices support critical operations or remote management depends on serial console reliability.

Technical view

The issue is in tty/xilinx_uartps SysRq handling. lockdep showed a circular dependency between cdns_uart_isr(), uart_port_lock(), printk(), console_lock(), and cdns_uart_console_write(). The fix splits SysRq preparation under the port lock from deferred handling after releasing it.

Likely exposure

Exposure is most likely on Linux systems that use the Xilinx UARTPS serial driver, common in Xilinx/Zynq-style embedded or appliance platforms. General Linux servers without this driver path are less likely exposed.

Exploitation context

The bundle does not show KEV listing, public exploitation, exploit code, or remote attack evidence. The described impact is a kernel deadlock, so the practical concern is availability rather than data theft.

Researcher notes

Evidence supports a lock-ordering deadlock fixed by restructuring SysRq handling. The source bundle lacks CVSS, CWE, exploitability details, and complete version-range semantics, so validation should rely on vendor kernel mapping.

Mitigation direction

  • Apply vendor or distribution kernel updates containing the referenced stable fixes.
  • Check Debian LTS guidance if running Debian-based affected kernels.
  • Prioritize embedded systems where Xilinx UARTPS is enabled or used as console.
  • If no packaged fix exists, track upstream stable kernel guidance before backporting.

Validation and detection

  • Inventory Linux kernel versions and enabled serial drivers on affected platforms.
  • Check whether the xilinx_uartps driver is built, loaded, or used for console.
  • Map running kernels to vendor advisories and upstream stable commits.
  • Review kernel logs for lockdep deadlock reports involving cdns_uart and console_lock.
Prepared
Confidence
medium
Sources
9

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2025-21820 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
1ADP providers
8Source links

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux74ea66d4ca061a3cd4c0e924e51b60e924644852, 74ea66d4ca061a3cd4c0e924e51b60e924644852, 74ea66d4ca061a3cd4c0e924e51b60e924644852, 74ea66d4ca061a3cd4c0e924e51b60e924644852, 74ea66d4ca061a3cd4c0e924e51b60e924644852, 74ea66d4ca061a3cd4c0e924e51b60e924644852unaffected
LinuxLinux4.6, 0, 5.15.179, 6.1.129, 6.6.78, 6.12.14, 6.13.3, 6.14affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.