Subject: CVE-2021-4440: x86/xen: Drop USERGS_SYSRET64 paravirt call

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

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

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

x86/xen: Drop USERGS_SYSRET64 paravirt call

commit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream.

USERGS_SYSRET64 is used to return from a syscall via SYSRET, but
a Xen PV guest will nevertheless use the IRET hypercall, as there
is no sysret PV hypercall defined.

So instead of testing all the prerequisites for doing a sysret and
then mangling the stack for Xen PV again for doing an iret just use
the iret exit from the beginning.

This can easily be done via an ALTERNATIVE like it is done for the
sysenter compat case already.

It should be noted that this drops the optimization in Xen for not
restoring a few registers when returning to user mode, but it seems
as if the saved instructions in the kernel more than compensate for
this drop (a kernel build in a Xen PV guest was slightly faster with
this patch applied).

While at it remove the stale sysret32 remnants.

  [ pawan: Brad Spengler and Salvatore Bonaccorso <[email-masked]>
	   reported a problem with the 5.10 backport commit edc702b4a820
	   ("x86/entry_64: Add VERW just before userspace transition").

	   When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in
	   syscall_return_via_sysret path as USERGS_SYSRET64 is runtime
	   patched to:

	.cpu_usergs_sysret64    = { 0x0f, 0x01, 0xf8,
				    0x48, 0x0f, 0x07 }, // swapgs; sysretq

	   which is missing CLEAR_CPU_BUFFERS. It turns out dropping
	   USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS
	   to be explicitly added to syscall_return_via_sysret path. Below
	   is with CONFIG_PARAVIRT_XXL=y and this patch applied:

	   syscall_return_via_sysret:
	   ...
	   <+342>:   swapgs
	   <+345>:   xchg   %ax,%ax
	   <+347>:   verw   -0x1a2(%rip)  <------
	   <+354>:   sysretq
  ]

The Linux kernel CVE team has assigned CVE-2021-4440 to this issue.

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

	Issue introduced in 5.10.215 with commit cea750c99d8f and fixed in 5.10.218 with commit 1424ab4bb386

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-2021-4440
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/entry/entry_64.S
	arch/x86/include/asm/irqflags.h
	arch/x86/include/asm/paravirt.h
	arch/x86/include/asm/paravirt_types.h
	arch/x86/kernel/asm-offsets_64.c
	arch/x86/kernel/paravirt.c
	arch/x86/kernel/paravirt_patch.c
	arch/x86/xen/enlighten_pv.c
	arch/x86/xen/xen-asm.S
	arch/x86/xen/xen-ops.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/1424ab4bb386df9cc590c73afa55f13e9b00dea2
	https://grsecurity.net/cve-2021-4440_linux_cna_case_study