Re: [PATCH 3/3] x86_32: Document our abuse of ss1 and sp1

[Denys Vlasenko <vda.linux@googlemail.com>]

On Tue, Mar 10, 2015 at 9:06 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> .ss1 also seems to be a write-only field:
>>
>> $ grep -r '[.>]ss1' .
>> ./include/asm/processor.h:      if (unlikely(tss->x86_tss.ss1 != thread->sysenter_cs)) {
>
> This is a read :)

You are right.

>> ./include/asm/processor.h:              tss->x86_tss.ss1 = thread->sysenter_cs;
>> ./include/asm/processor.h:              .ss1            = __KERNEL_CS,                            \
>> ./kernel/cpu/common.c:  tss->x86_tss.ss1 = __KERNEL_CS;

>>> +      * but we need to context switch it because we do
>>> +      * horrible things to the kernel stack in vm86 mode.
>>> +      *
>>> +      * We use SYSENTER_CS to disable sysenter in vm86 mode to avoid
>>> +      * corrupting the stack if we went through the sysenter path
>>> +      * from vm86 mode.
>>> +      */
>>
>> I'm confused how loading ss1/sp1 with anything can disable sysenter.
>> SYSENTER insn does not use those fields.
>>
>> What you _can_ disable is you can make it impossible to enter RING1
>> if tss.ss1 is invalid.
>
> Does it make sense now that I pointed out the read of ss1?  If not,
> I'll improve the comments.

I propose the following comment about tss.ss1:

/*
tss.ss1 is used to avoid redundant wrmsr(MSR_IA32_SYSENTER_CS).
After wrmsr, tss.ss1 is set to the written value. If on future task switches
tss.ss1 already contains the value to be written, wrmsr is skipped.
*/


>>> +      * We use SYSENTER_CS to disable sysenter in vm86 mode to avoid
>>> +      * corrupting the stack if we went through the sysenter path
>>> +      * from vm86 mode.

This appears to be untrue - SYSENTER_CS isn't used to disable sysenter.
Zero is.

Disabling sysenter happens in vm86_32.c here, by setting it to 0:

static void do_sys_vm86(struct kernel_vm86_struct *info, struct
task_struct *tsk)
{
...
        if (cpu_has_sep)
                tsk->thread.sysenter_cs = 0;
        load_sp0(tss, &tsk->thread);   <-- this sets
wrmsr(MSR_IA32_SYSENTER_CS, 0);
...