Bug 2322309 (CVE-2024-50072)

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. ]