In June 2024, a subtle but potentially dangerous bug was fixed in the Linux kernel that affected stack object detection, especially when advanced memory debugging features were enabled. This bug, now tracked as CVE-2024-53128, could cause the kernel to misidentify memory locations, leading to confusing warnings and possible issues with kernel debugging. Let's break down what happened, why it mattered, and how it's fixed—with code snippets, example logs, and original references.

1. What Happened? (Layman's Terms)

Linux has strong protection and debugging tools for memory, including KASAN (Kernel Address Sanitizer) and DEBUG_OBJECTS. When certain advanced options (CONFIG_KASAN_SW_TAGS, CONFIG_KASAN_STACK, and CONFIG_DEBUG_OBJECTS) are turned on, a kernel function called object_is_on_stack() got tripped up by KASAN's clever "pointer tagging".

The bug:
KASAN tags pointers for tracking, but the stack pointer itself isn't tagged. When object_is_on_stack() checked if a pointer (with a tag) was on the stack (plain pointer), the result was wrong. This could make the kernel think a stack object *wasn't on the stack*, sometimes flooding kernel logs with warnings.

For kernel developers: False warnings make debugging real issues much harder.

- For security: Confusing stack detection can hide or create faults that attackers might try to exploit.

When the object_is_on_stack() misbehaved, you could see warnings like this on your console or logs

ODEBUG: object 3eff800082ea7bb is NOT on stack ffff800082ea000, but annotated.
------------[ cut here ]------------
WARNING: CPU:  PID: 1 at lib/debugobjects.c:557 __debug_object_init+x330/x364
...
Call trace:
 __debug_object_init+x330/x364
 debug_object_init_on_stack+x30/x3c
 schedule_hrtimeout_range_clock+xac/x26c
 schedule_hrtimeout+x1c/x30
 wait_task_inactive+x1d4/x25c
 kthread_bind_mask+x28/x98
 init_rescuer+x1e8/x280
 workqueue_init+x1a/x3cc
 kernel_init_freeable+x118/x200
 kernel_init+x28/x1f
 ret_from_fork+x10/x20
---[ end trace 000000000000000 ]---

This wasn't just noise: it pointed to real confusion in the kernel's understanding of its own stack memory.

Here's a snippet showing the logic involved (simplified for clarity)

// Before the fix:
bool object_is_on_stack(const void *obj)
{
    // 'obj' could have a tag (extra bits) from KASAN
    // 'stack_base' is plain (untagged)
    return obj >= stack_base && obj < stack_base + THREAD_SIZE;
}

If obj had a tag, say the pointer looked like 3eff800082ea7bb, but stack_base was ffff800082ea000, the comparison failed—even if they pointed to the same place!

5. The Fix: Untagging the Pointer

To make object_is_on_stack() work right, the kernel must strip KASAN tags before comparing addresses. The new code does just that:

// After the fix (real code from patch):
#include <linux/kasan.h>

bool object_is_on_stack(const void *obj)
{
#ifdef CONFIG_KASAN_SW_TAGS
    obj = kasan_reset_tag(obj); // Remove the tag bits!
#endif
    return obj >= stack_base && obj < stack_base + THREAD_SIZE;
}

kasan_reset_tag() clears the tag bits, making the pointer safe to compare.

6. Is This an Exploit or Just a Bug?

CVE-2024-53128 itself doesn't let attackers break into systems directly. It's a logic bug that breaks diagnostics.
But:

Confused diagnostics = harder for developers to spot *real* vulnerabilities.

- If attackers find ways to leverage faulty debug info, things can get dangerous—especially in custom or embedded kernels.

7. Exploitability Details

Status:

Not directly exploitable for privilege escalation or code execution.

- Could hide/hinder detection of other memory errors, making security work harder.

Proof-of-concept (PoC):

Enable a kernel with KASAN tagging, KASAN stack, and debug objects

CONFIG_KASAN_SW_TAGS=y
CONFIG_KASAN_STACK=y
CONFIG_DEBUG_OBJECTS=y

Run a workload (boot is enough); observe kernel logs for "ODEBUG: object ... is NOT on stack ..." lines.
No actual payload needed—simply building and booting the kernel with these config flags can trigger the bug.

8. References & Patch

- Patch (commit): sched/task_stack: fix object_is_on_stack() for KASAN tagged pointers (kernel.org)
- KASAN Documentation: kernel.org: KASAN
- Debug Objects: kernel.org: debugobjects
- CVE Details: *(will appear at cve.org/CVERecord?id=CVE-2024-53128 soon)*

9. Summary Table

| Aspect | Details |
|---------------------|-------------------------------------------------------------------------|
| Affected Versions | Linux 6.12-rc and possibly earlier (with specific config combinations) |
| Patch Available | Yes, mainline as of June 4, 2024 |
| Trigger | Enable KASAN tagging, KASAN stack, and Debug Objects |
| Exploitability | None direct, can hinder kernel crash & memory bug detection |
| Symptom | "ODEBUG: object ... is NOT on stack ..." + kernel warnings |

10. Final Word: Should You Worry?

If you run vanilla Linux kernels with KASAN and debug options, pull the latest patches now!
For typical users and prod servers, this won't affect you unless you're doing deep kernel/hardware debugging.

Want to dig deeper? Check out the official commit patch or Linux KASAN documentation.


Stay safe and patched! <br>— Linux Kernel Security Watch

*(This article is original content produced in June 2024. Attribution for excerptions: Linux kernel project.)*

Timeline

Published on: 12/04/2024 15:15:12 UTC
Last modified on: 12/19/2024 09:39:53 UTC