CVE-2024-26706

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.