Subject: CVE-2024-50072: x86/bugs: Use code segment selector for VERW operand

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

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

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

x86/bugs: Use code segment selector for VERW operand

Robert Gill reported below #GP in 32-bit mode when dosemu software was
executing vm86() system call:

  general protection fault: 0000 [#1] PREEMPT SMP
  CPU: 4 PID: 4610 Comm: dosemu.bin Not tainted 6.6.21-gentoo-x86 #1
  Hardware name: Dell Inc. PowerEdge 1950/0H723K, BIOS 2.7.0 10/30/2010
  EIP: restore_all_switch_stack+0xbe/0xcf
  EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
  ESI: 00000000 EDI: 00000000 EBP: 00000000 ESP: ff8affdc
  DS: 0000 ES: 0000 FS: 0000 GS: 0033 SS: 0068 EFLAGS: 00010046
  CR0: 80050033 CR2: 00c2101c CR3: 04b6d000 CR4: 000406d0
  Call Trace:
   show_regs+0x70/0x78
   die_addr+0x29/0x70
   exc_general_protection+0x13c/0x348
   exc_bounds+0x98/0x98
   handle_exception+0x14d/0x14d
   exc_bounds+0x98/0x98
   restore_all_switch_stack+0xbe/0xcf
   exc_bounds+0x98/0x98
   restore_all_switch_stack+0xbe/0xcf

This only happens in 32-bit mode when VERW based mitigations like MDS/RFDS
are enabled. This is because segment registers with an arbitrary user value
can result in #GP when executing VERW. Intel SDM vol. 2C documents the
following behavior for VERW instruction:

  #GP(0) - If a memory operand effective address is outside the CS, DS, ES,
	   FS, or GS segment limit.

CLEAR_CPU_BUFFERS macro executes VERW instruction before returning to user
space. Use %cs selector to reference VERW operand. This ensures VERW will
not #GP for an arbitrary user %ds.

[ mingo: Fixed the SOB chain. ]

The Linux kernel CVE team has assigned CVE-2024-50072 to this issue.

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

	Issue introduced in 6.6.21 with commit ca13d8cd8dac and fixed in 6.6.58 with commit 481b477ab63c
	Issue introduced in 6.8 with commit a0e2dab44d22 and fixed in 6.11.5 with commit bc576fbaf82d
	Issue introduced in 6.8 with commit a0e2dab44d22 and fixed in 6.12-rc4 with commit e4d210201854
	Issue introduced in 5.10.215 with commit 50f021f0b985
	Issue introduced in 5.15.154 with commit d54de9f2a127
	Issue introduced in 6.1.81 with commit 2e3087505ddb
	Issue introduced in 6.7.9 with commit 51eca9f1fd04

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-2024-50072
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/include/asm/nospec-branch.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/481b477ab63c7245715a3e57ba79eb87c2dc0d02
	https://git.kernel.org/stable/c/bc576fbaf82deded606e69a00efe9752136bf91d
	https://git.kernel.org/stable/c/e4d2102018542e3ae5e297bc6e229303abff8a0f