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.