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

From: Greg Kroah-Hartman <[email-masked]>

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.

The Linux kernel CVE team has assigned CVE-2024-26706 to this issue.

Affected and fixed versions
===========================

	Fixed in 6.1.79 with commit 23027309b099
	Fixed in 6.6.18 with commit fa69a8063f8b
	Fixed in 6.7.6 with commit ce31d79aa1f1
	Fixed in 6.8 with commit 8b1d72395635

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26706
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/parisc/Kconfig
	arch/parisc/include/asm/assembly.h
	arch/parisc/include/asm/extable.h
	arch/parisc/include/asm/special_insns.h
	arch/parisc/include/asm/uaccess.h
	arch/parisc/kernel/cache.c
	arch/parisc/kernel/unaligned.c
	arch/parisc/mm/fault.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/23027309b099ffc4efca5477009a11dccbdae592
	https://git.kernel.org/stable/c/fa69a8063f8b27f3c7434a0d4f464a76a62f24d2
	https://git.kernel.org/stable/c/ce31d79aa1f13a2345791f84935281a2c194e003
	https://git.kernel.org/stable/c/8b1d72395635af45410b66cc4c4ab37a12c4a831