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

[Peter Zijlstra]

On Fri, Feb 15, 2019 at 08:06:56PM -0800, hpa@xxxxxxxxx wrote:
> This implies we invoke schedule -- a restricted operation (consider
> may_sleep) during execution of STAC-enabled code, but *not* as an
> exception or interrupt, since those preserve the flags.

Meet preempt_enable().

> I have serious concerns about this. This is more or less saying that
> we have left an unlimited gap and have had AC escape.

Yes; by allowing normal C in between STAC and CLAC this is so.

> Does *anyone* see a need to allow this? I got a question at LPC from
> someone about this, and what they were trying to do once all the
> layers had been unwound was so far down the wrong track for a root
> problem that actually has a very simple solution.

Have you read the rest of the thread?

All it takes for this to explode is a call to a C function that has
tracing on it in between the user_access_begin() and user_access_end()
things. That is a _very_ easy thing to do.

Heck, GCC is allowed to generate that broken code compiling
lib/strnlen_user.c; it doesn't _have_ to inline do_strnlen_user (esp.
with CONFIG_OPTIMIZE_INLINING), and making that a function call would
get us fentry hooks and ftrace and *BOOM*.

(Now, of course, its a static function with a single caller, and GCC
isn't _THAT_ stupid, but it could, if it wanted to)

Since the bar is _that_ low for mistakes, I figure we'd better fix it.