CVE-2026-72215: MIPS: DEC: Ensure 32-bit stack location for o32 prom_printf()

CVE-2026-72215: MIPS: DEC: Ensure 32-bit stack location for o32 prom_printf()

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

MIPS: DEC: Ensure 32-bit stack location for o32 prom_printf()

In 64-bit configurations calling any firmware entry points from a kernel
thread other than the initial one will result in a situation where the
stack has been placed in the XKPHYS 64-bit memory segment.

Consequently the stack pointer is no longer a 32-bit value and when the
32-bit firmware code called uses 32-bit ALU operations to manipulate the
stack pointer, the calculated result is incorrect (in fact in the 64-bit
MIPS ISA almost all 32-bit ALU operations will produce an unpredictable
result when executed on 64-bit data) and control goes astray.

This may happen when no final console driver has been enabled in the
configuration and consequently the initial console continues being used
late into bootstrap, or with an upcoming change that will switch the zs
driver to use a platform device, which in turn will make the console
handover happen only after other kernel threads have already been
started, and the kernel will hang at:

  pid_max: default: 32768 minimum: 301

or somewhat later, but always before:

  cblist_init_generic: Setting adjustable number of callback queues.

has been printed.

It seems that only the prom_printf() entry point is affected.  Of all
the other entry points wired only rex_slot_address() and rex_gettcinfo()
are called from a kernel thread other than the initial one, specifically
kernel_init(), and they are leaf functions that do no business with the
stack, having worked with no issue ever since 64-bit support was added
for the platform back in 2002.

To address this issue then, arrange for the stack to be switched in the
o32 wrapper as required for prom_printf() only, by supplying call_o32()
with a pointer to a chunk of initdata space, which is placed in the
CKSEG0 32-bit compatibility segment, observing that prom_printf() is
only called from console output handler and therefore with the console
lock held, implying no need for this code to be reentrant.

Other firmware entry points may be called with interrupts enabled and no
lock held, and may therefore require that call_o32() be reentrant.  They
trigger no issue at this point and "if it ain't broke, don't fix it," so
just leave them alone.

The Linux kernel CVE team has assigned CVE-2026-72215 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.10.261 with commit 9cd4a8ed12d2a6313592e43c14cca5eca6717bee
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.15.212 with commit 3394757c5d3970ab36d2aafa6dd40952b43f0d16
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.1.178 with commit 1ed18b5c3be9657fe288fa4dae0bef2470598683
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.6.145 with commit 27857db30c98583e12dd8d939ba2370bce08a5be
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.12.97 with commit d828bca84311e278093e60b2864a54f4bbb0c2ad
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.18.40 with commit d3fd2d358df0ca712183509cbbed6a16bde1d17e
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.1.5 with commit 3ae86630b94f72bf4012c6322f81f622dc4fdf24
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2-rc1 with commit 5ff79e8bdc75db51e30298a75939e2308e7658e0

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-2026-72215
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/mips/dec/prom/init.c
	arch/mips/include/asm/dec/prom.h


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/9cd4a8ed12d2a6313592e43c14cca5eca6717bee
	https://git.kernel.org/stable/c/3394757c5d3970ab36d2aafa6dd40952b43f0d16
	https://git.kernel.org/stable/c/1ed18b5c3be9657fe288fa4dae0bef2470598683
	https://git.kernel.org/stable/c/27857db30c98583e12dd8d939ba2370bce08a5be
	https://git.kernel.org/stable/c/d828bca84311e278093e60b2864a54f4bbb0c2ad
	https://git.kernel.org/stable/c/d3fd2d358df0ca712183509cbbed6a16bde1d17e
	https://git.kernel.org/stable/c/3ae86630b94f72bf4012c6322f81f622dc4fdf24
	https://git.kernel.org/stable/c/5ff79e8bdc75db51e30298a75939e2308e7658e0