CVE-2024-26841 in Linux
Sumário
de VulDB • 29/06/2026
Based on the kernel crash dump provided, here is a detailed analysis of the issue.
### **Summary** The system has encountered an **Illegal Instruction Exception (Breakpoint)** while executing CPU hotplug code (`cpuhp`) during CPU deactivation. The process involved is `cpuhp/6` (CPU online/offline state machine thread for logical CPU 6). The crash occurs in the kernel function `__static_key_slow_dec_cpuslocked`, which suggests a problem with **jump labels** or **branch prediction optimization structures** when modifying code paths dynamically during CPU shutdown.
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### **Key Information from Dump** 1. **Exception Status (ECFG/ESTAT):** - `LIE=2-4,10-12`: Indicates an exception related to instruction fetch or execution. - `ECode=12 EsubCode=0`: On LoongArch architecture, Exception Code 12 typically corresponds to **`Breakpoint Instruction`** (`break`) or an illegal opcode if the breakpoint was not properly handled by a debugger/tracer.
2. **CPU & Process:** - **CPU:** Logical CPU 6 (PID 45). - **Process:** `cpuhp/6` – This is the kernel thread responsible for managing CPU hotplug states (online/offline transitions). - **Kernel Version:** `6.8.0-rc5+ #1340` – A recent release candidate, possibly with custom patches (`Loongson-3A6000-HV`).
3. **Call Trace Analysis:** ``` [] show_stack+0x48/0x1a0
[] dump_stack_lvl+0x78/0xa0
[] __warn+0x90/0x1a0 <-- Kernel detected a warning condition and triggered this trace.
[] report_bug+0x1b8/0x280 <-- Reported as a bug (likely BUG_ON or WARN_ON).
[] do_bp+0x264/0x420 <-- "do_breakpoint" handler executed. This confirms the CPU hit a `break` instruction intentionally placed by kernel code to signal an error condition.
[] __static_key_slow_dec_cpuslocked+0xec/0x100 <-- The function where the breakpoint was triggered.
[] sched_cpu_deactivate+0x2fc/0x300 <-- Called from CPU deactivation logic.
```
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### **Root Cause Analysis**
#### **1. Why `__static_key_slow_dec_cpuslocked`?** - Static keys (jump labels) are used in the Linux kernel to optimize rarely-taken branches by replacing them with NOPs or jumps at runtime. - The function `__static_key_slow_dec_cpuslocked` is called when a static key's counter drops, potentially disabling an optimized code path. - **Critical Context:** This happens during CPU deactivation (`sched_cpu_deactivate`). When a CPU goes offline, the kernel must ensure that no other CPUs are executing code paths dependent on this CPU’s state or that rely on per-CPU data structures being modified safely.
#### **2. Why `do_bp` (Breakpoint)?** - The kernel uses `break` instructions as intentional traps for debugging and error reporting via `WARN_ON()` or `BUG_ON()`. - In the context of static keys, a breakpoint here likely indicates: - A **race condition**: Another CPU might still be executing code that depends on this static key being enabled while it is being disabled. - **Invalid state transition:** The kernel detected an inconsistent state in the static key counter or related per-CPU data during hotplug. - **Architecture-specific bug:** LoongArch may have a specific issue with how jump labels are patched or synchronized across CPUs, especially on newer chips like the 3A6000-HV.
#### **3. Why During CPU Hotplug?** - CPU hotplug is complex because it involves synchronizing all other CPUs to ensure they don’t access per-CPU data that’s being freed or modified. - If a static key controls a path that accesses per-CPU variables, and one CPU goes offline while another is in the middle of updating/decrementing that key, corruption can occur.
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### **Possible Causes** 1. **Kernel Bug in Static Key Handling:** A race condition between `sched_cpu_deactivate` and static key updates was not properly handled in kernel 6.8-rc5. 2. **LoongArch-Specific Issue:** The Loongson 3A6000-HV might have subtle timing or cache coherency issues with jump label patching that aren’t present on x86/ARM. 3. **Corrupted Memory/Page Table:**
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