Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

[Andy Lutomirski]

> On Feb 18, 2019, at 4:24 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
>> On Mon, Feb 18, 2019 at 2:31 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> 
>> The question is what "fix it" means. I'm really concerned about AC escapes,
>> and everyone else should be, too.
> 
> I do think that it might be the right thing to do to add some kind of
> WARN_ON_ONCE() for AC being set in various can-reschedule situations.
> 
> We'd just have to abstract it sanely. I'm sure arm64 has the exact
> same issue with PAN - maybe it saves properly, but the same "we
> wouldn't want to go through the scheduler with PAN clear".
> 
> On x86, we might as well check DF at the same time as AC.
> 
> 

hpa is right, though â calling into tracing code with AC set is not really so good.  And calling schedule() (via preempt_enable() or whatever) is also bad because it runs all the scheduler code with AC on.  Admittedly, the scheduler is not *that* interesting of an attack surface.