x86/bugs: Use code segment selector for VERW operand

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. ]