CVE-2024-26706 in Linux
Summary
by MITRE • 04/03/2024
In the Linux kernel, the following vulnerability has been resolved:
parisc: Fix random data corruption from exception handler
The current exception handler implementation, which assists when accessing user space memory, may exhibit random data corruption if the compiler decides to use a different register than the specified register %r29 (defined in ASM_EXCEPTIONTABLE_REG) for the error code. If the compiler choose another register, the fault handler will nevertheless store -EFAULT into %r29 and thus trash whatever this register is used for. Looking at the assembly I found that this happens sometimes in emulate_ldd().
To solve the issue, the easiest solution would be if it somehow is possible to tell the fault handler which register is used to hold the error code. Using %0 or %1 in the inline assembly is not posssible as it will show up as e.g. %r29 (with the "%r" prefix), which the GNU assembler can not convert to an integer.
This patch takes another, better and more flexible approach: We extend the __ex_table (which is out of the execution path) by one 32-word. In this word we tell the compiler to insert the assembler instruction "or %r0,%r0,%reg", where %reg references the register which the compiler choosed for the error return code. In case of an access failure, the fault handler finds the __ex_table entry and can examine the opcode. The used register is encoded in the lowest 5 bits, and the fault handler can then store -EFAULT into this register.
Since we extend the __ex_table to 3 words we can't use the BUILDTIME_TABLE_SORT config option any longer.
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Analysis
by VulDB Data Team • 08/03/2025
The vulnerability described in CVE-2024-26706 represents a critical flaw in the Linux kernel's exception handling mechanism specifically affecting the parisc architecture. This issue stems from an improper implementation of the exception table handling code that manages user space memory access operations. The flaw manifests when the compiler optimizes code generation and selects a different register than the designated %r29 register to hold the error code during exception processing. This misalignment causes random data corruption because the fault handler consistently attempts to store the -EFAULT error code into %r29 regardless of which register the compiler actually uses for the error return value.
The technical root cause of this vulnerability lies in the interaction between the kernel's exception handling infrastructure and compiler optimization behaviors. When the emulate_ldd() function encounters memory access violations, the fault handler's hardcoded assumption about register usage breaks down. This creates a scenario where legitimate register contents get overwritten with error codes, leading to unpredictable system behavior and potential data corruption. The vulnerability is particularly concerning because it operates at the kernel level where such corruption can lead to system instability, data loss, or even privilege escalation opportunities. This type of flaw aligns with CWE-129, which covers improper handling of buffer indices and array bounds, and also relates to CWE-787, concerning out-of-bounds write conditions.
The operational impact of this vulnerability extends beyond simple data corruption to potentially compromise system integrity and availability. Attackers could exploit this weakness to cause system crashes, data inconsistencies, or manipulate kernel memory structures through carefully crafted memory access patterns. The vulnerability affects systems running Linux kernels on parisc architecture, making it particularly relevant for enterprise servers and legacy systems that rely on this processor family. Given that the fault handler operates in critical execution paths during memory access operations, any corruption could propagate through the system's memory management subsystem, potentially affecting multiple processes and applications running concurrently.
The proposed mitigation strategy involves extending the __ex_table structure to accommodate additional metadata about register usage during exception handling. This solution introduces a more flexible approach by encoding register information within the exception table entries themselves, allowing the fault handler to dynamically determine which register contains the error code. The implementation uses a modified assembly instruction format that encodes the actual register used by the compiler in the lowest 5 bits of the opcode, enabling the fault handler to properly store error codes into the correct register. This approach represents a sophisticated solution that addresses the fundamental mismatch between static kernel assumptions and dynamic compiler optimizations. However, the patch requires disabling the BUILDTIME_TABLE_SORT configuration option, indicating a trade-off between performance optimization and the robustness of the exception handling mechanism. The solution aligns with ATT&CK technique T1068 by potentially enabling privilege escalation through kernel memory corruption, and demonstrates the importance of proper exception handling in maintaining system security and stability.