# CVE-2024-26706: parisc: Fix random data corruption from exception handler

CVE-2024-26706 is fixed in linux on 1 platform, first by DEBIAN-CVE-2024-26706, released April 3, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2024-26706
- Priority: Routine
- CVSS: 5.5 (Moderate)
- EPSS: 0.0027
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 3, 2024
- Weakness: Out-of-bounds Write
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-26706

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.82-1 | [DEBIAN-CVE-2024-26706](https://patchblox.com/security/data/fix/linux/debian-cve-2024-26706.md) | Apr 3, 2024 |
| Linux | linux Debian 13.5 (trixie) | 6.7.7-1 | [DEBIAN-CVE-2024-26706](https://patchblox.com/security/data/fix/linux/debian-cve-2024-26706.md) | Apr 3, 2024 |
| Linux | linux Debian 14 | 6.7.7-1 | [DEBIAN-CVE-2024-26706](https://patchblox.com/security/data/fix/linux/debian-cve-2024-26706.md) | Apr 3, 2024 |

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