Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

[Denys Vlasenko]

On Fri, Apr 24, 2015 at 10:21 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Apr 23, 2015 at 7:15 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET
>> with SS == 0 results in an invalid usermode state in which SS is
>> apparently equal to __USER_DS but causes #SS if used.
>>
>> Work around the issue by replacing NULL SS values with __KERNEL_DS
>> in __switch_to, thus ensuring that SYSRET never happens with SS set
>> to NULL.
>>
>> This was exposed by a recent vDSO cleanup.
>>
>> Fixes: e7d6eefaaa44 x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
>>
>> Tested only on Intel, which isn't very interesting.  I'll tidy up
>> and send a test case, too, once Borislav confirms that it works.
>>
>> Please don't actually apply this until we're sure we understand the
>> scope of the issue.  If this doesn't affect SYSRETQ, then we might
>> to fix it on before SYSRETL to avoid impacting 64-bit processes
>> at all.
>>
>
> After sleeping on it, I think I want to offer a different, more
> complicated approach.  AFAIK there are really only two ways that this
> issue can be visible:
>
> 1. SYSRETL.  We can fix that up in the AMD SYSRETL path.  I think
> there's a decent argument that that path is less performance-critical
> than context switches.
>
> 2. SYSRETQ.  The only way that I know of to see the problem is SYSRETQ
> followed by a far jump or return.  This is presumably *extremely*
> rare.
>
> What if we fixed #2 up in do_stack_segment.  We should double-check
> the docs, but I think that this will only ever manifest as #SS(0) with
> regs->ss == __USER_DS and !user_mode_64bit(regs).  We need to avoid
> infinite retry looks, but this might be okay.  I think that #SS(0)
> from userspace under those conditions can *only* happen as a result of
> this issue.  Even if not, we could come up with a way to only retry
> once per syscall (e.g. set some ti->status flag in the 64-bit syscall
> path on AMD and clear it in do_stack_segment).
>
> This might be way more trouble than it's worth.

Exactly my feeling. What are you trying to save? About four CPU
cycles of checking %ss != __KERNEL_DS on each switch_to?
That's not worth bothering about. Your last patch seems to be perfect.
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