Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand

On 10/8/24 23:11, Borislav Petkov wrote:

...

.macro CLEAR_CPU_BUFFERS

• ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF

+#ifdef CONFIG_X86_64

• ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF

+#else

• /*

• * In 32bit mode, the memory operand must be a %cs reference. The data
  

• * segments may not be usable (vm86 mode), and the stack segment may not
  

• * be flat (ESPFIX32).
  

• */
  

• ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF

+#endif

So why didn't we ifdef the "verw mds_verw_sel(%rip)" and "verw %cs:mds_verw_sel" macro argument instead of adding more bigger ugly ifdeffery?

I'm not jumping for joy about how it looks, but I applied it because it's good enough and the regression was about to get its driver's license. ;)

I did spend some time noodling to come up with _some_ common 32/64-bit implementation, but 32-bit is just too special of a snowflake.