CVE-2025-21656 - How a Linux Kernel Drivetemp Bug Gave Garbage Data When Drives Glitched

---

Summary:
A subtle bug once hid in the Linux kernel’s drivetemp hardware monitoring driver (hwmon). If your storage drive glitched or disconnected, instead of a neat failure, the driver sometimes handed you complete garbage data. This post explains the vulnerability CVE-2025-21656, why it happened, how it was fixed, and what it could mean for you.

What Is The Problem?

In the Linux kernel, the drivetemp driver helps monitor the temperature of ATA/SATA drives via SCSI commands. It talks to drives and returns their temperature data for health monitoring.

But when a SCSI error happened—like when a drive was disconnected or misbehaved—the code didn’t always react right. Instead of safely signaling an error, the driver sometimes delivered *uninitialized* or *invalid* data up to the monitoring system (userspace, monitoring tools, scripts, etc).

This is serious, as users and automated tools may have acted on “trash data”—wrong temperatures, risk assessments, alerts, and log entries could all be triggered incorrectly.

Negative error codes: Standard Linux error reporting ("something failed")

- Positive error codes: SCSI command result fields (not necessarily an error by Linux convention, but definitely an error in hardware terms).

But the drivetemp driver only checked for *negative* codes. If a *positive* error code came back, those errors were just passed back as if no serious error occurred. The higher-level hwmon core (which only understands negative codes) thus treated these as "results," and whatever junk happened to be in the data buffer became the reported "temperature."

Real Life Example

Imagine you just unplugged a SATA drive. The next time the OS polls its temperature via the drivetemp driver, SCSI returns a positive error as “this drive doesn’t exist!” The old code WOULD NOT flag this, and the hardware monitoring system just believed whatever data was already sitting in memory.

That could be random junk—like a temperature of , 300, -200, or anything else.

The Patch & The Fix

The fix is simple:
If scsi_execute_cmd() returns a positive value, treat it as a hard error (return -EIO)—don’t pass it to hwmon as legitimate data.

Patch Excerpt

static int drivetemp_scsi_read(struct device *dev, u8 *buffer, int len)
{
    int ret;
    ret = scsi_execute_cmd(...);

    if (ret > )      // <-- Fix: check for positive error codes!
        ret = -EIO;   // Return "input/output error" to upper layers
    // rest of function...
    return ret;
}

Key lines:
- If the return code (ret) is positive, set it to -EIO (standard Linux I/O error).
- This ensures hwmon core recognizes this as an error and won't propagate uninitialized temperature data.

Full patch and technical details:
- Linux Kernel Patch – hwmon: (drivetemp) Fix driver producing garbage data when SCSI errors occur
- CVE Record for CVE-2025-21656 *(link will populate as CVE assignment becomes official)*

Potential Exploit Impact

This isn’t a classic remote code exec, privilege escalation, or attacker’s backdoor—so is it just a bug?

Why it matters

- Security software/monitoring: Admins relying on hardware health data for security or reliability could be misled. Automated responses (like spin-downs, warnings, or server shutdowns) might go off at random.
- Data loss or downtime: Systems might take unnecessary action (or, worse, no action) based on false status, e.g., thinking a healthy drive is burning up, or a failed drive is cool and healthy.
- Data integrity: Automation scripts that log drive health, or perform predictive maintenance, could create false histories and take wrong steps.

If an attacker or script could cause drive disconnect/reconnects in sync with hardware polling, they *might* goad misbehavior elsewhere, though this is a stretch. Still: it’s a data integrity/availability concern.

Are you affected?

- Any Linux kernel before the above patch (expected in 6.10+) is vulnerable. Common in servers using hwmon for drive temperature.

Disable drivetemp if you do not use drive temperature monitoring and cannot upgrade.

- Monitor your monitoring software: Fuzz or test with unplugged drives to see what’s reported in your stack.

Conclusion

CVE-2025-21656 reminds us: sometimes a kernel error isn’t a crash or exploit—it’s bogus environmental data propagating upwards, sowing quiet chaos.

If you watch storage health with Linux tools, make sure your system isn’t fooled by drive errors. The best monitoring is only as honest as its drivers.

Stay patched. Stay sane.

Timeline

Published on: 01/21/2025 13:15:09 UTC
Last modified on: 05/04/2025 07:18:21 UTC