Subject: CVE-2024-35803: x86/efistub: Call mixed mode boot services on the firmware's stack

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

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

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

x86/efistub: Call mixed mode boot services on the firmware's stack

Normally, the EFI stub calls into the EFI boot services using the stack
that was live when the stub was entered. According to the UEFI spec,
this stack needs to be at least 128k in size - this might seem large but
all asynchronous processing and event handling in EFI runs from the same
stack and so quite a lot of space may be used in practice.

In mixed mode, the situation is a bit different: the bootloader calls
the 32-bit EFI stub entry point, which calls the decompressor's 32-bit
entry point, where the boot stack is set up, using a fixed allocation
of 16k. This stack is still in use when the EFI stub is started in
64-bit mode, and so all calls back into the EFI firmware will be using
the decompressor's limited boot stack.

Due to the placement of the boot stack right after the boot heap, any
stack overruns have gone unnoticed. However, commit

  5c4feadb0011983b ("x86/decompressor: Move global symbol references to C code")

moved the definition of the boot heap into C code, and now the boot
stack is placed right at the base of BSS, where any overruns will
corrupt the end of the .data section.

While it would be possible to work around this by increasing the size of
the boot stack, doing so would affect all x86 systems, and mixed mode
systems are a tiny (and shrinking) fraction of the x86 installed base.

So instead, record the firmware stack pointer value when entering from
the 32-bit firmware, and switch to this stack every time a EFI boot
service call is made.

The Linux kernel CVE team has assigned CVE-2024-35803 to this issue.

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

	Fixed in 6.1.84 with commit 2149f8a56e2e
	Fixed in 6.6.24 with commit 930775060ca3
	Fixed in 6.7.12 with commit fba7ee718758
	Fixed in 6.8.3 with commit 725351c03645
	Fixed in 6.9 with commit cefcd4fe2e3a

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-2024-35803
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/x86/boot/compressed/efi_mixed.S

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/2149f8a56e2ed345c7a4d022a79f6b8fc53ae926
	https://git.kernel.org/stable/c/930775060ca348b8665f60eef14b204172d14f31
	https://git.kernel.org/stable/c/fba7ee7187581b5bc222003e73e2592b398bb06d
	https://git.kernel.org/stable/c/725351c036452b7db5771a7bed783564bc4b99cc
	https://git.kernel.org/stable/c/cefcd4fe2e3aaf792c14c9e56dab89e3d7a65d02