Subject: CVE-2024-42126: powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

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

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

nmi_enter()/nmi_exit() touches per cpu variables which can lead to kernel
crash when invoked during real mode interrupt handling (e.g. early HMI/MCE
interrupt handler) if percpu allocation comes from vmalloc area.

Early HMI/MCE handlers are called through DEFINE_INTERRUPT_HANDLER_NMI()
wrapper which invokes nmi_enter/nmi_exit calls. We don't see any issue when
percpu allocation is from the embedded first chunk. However with
CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there are chances where percpu
allocation can come from the vmalloc area.

With kernel command line "percpu_alloc=page" we can force percpu allocation
to come from vmalloc area and can see kernel crash in machine_check_early:

[    1.215714] NIP [c000000000e49eb4] rcu_nmi_enter+0x24/0x110
[    1.215717] LR [c0000000000461a0] machine_check_early+0xf0/0x2c0
[    1.215719] --- interrupt: 200
[    1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable)
[    1.215722] [c000000fffd731b0] [0000000000000000] 0x0
[    1.215724] [c000000fffd73210] [c000000000008364] machine_check_early_common+0x134/0x1f8

Fix this by avoiding use of nmi_enter()/nmi_exit() in real mode if percpu
first chunk is not embedded.

The Linux kernel CVE team has assigned CVE-2024-42126 to this issue.

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

	Fixed in 5.10.224 with commit fb6675db04c4
	Fixed in 5.15.165 with commit e2afb26615ad
	Fixed in 6.1.98 with commit 8d3f83dfb236
	Fixed in 6.6.39 with commit 0f37946c62c4
	Fixed in 6.9.9 with commit 2c78c9411e68
	Fixed in 6.10 with commit 0db880fc865f

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-2024-42126
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/powerpc/include/asm/interrupt.h
	arch/powerpc/include/asm/percpu.h
	arch/powerpc/kernel/setup_64.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/fb6675db04c4b79883373edc578d5df7bbc84848
	https://git.kernel.org/stable/c/e2afb26615adf6c3ceaaa7732aa839bcd587a057
	https://git.kernel.org/stable/c/8d3f83dfb23674540c827a8d65fba20aa300b252
	https://git.kernel.org/stable/c/0f37946c62c48a907625348cbc720a7a0c547d1e
	https://git.kernel.org/stable/c/2c78c9411e685dbc9eac8c2845111b03501975b8
	https://git.kernel.org/stable/c/0db880fc865ffb522141ced4bfa66c12ab1fbb70