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

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Date:   Wed, 20 Feb 2019 14:55:59 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

On 2/19/19 4:48 AM, Will Deacon wrote:
> 
> I think you'll still hate this, but could we not disable preemption during
> the uaccess-enabled region, re-enabling it on the fault path after we've
> toggled uaccess off and disable it again when we return back to the
> uaccess-enabled region? Doesn't help with tracing, but it should at least
> handle the common case.
> 

There is a worse problem with this, I still realize: this would mean blocking
preemption across what could possibly be a *very* large copy_from_user(), for
example.

Exceptions *have* to handle this; there is no way around it. Perhaps the
scheduler isn't the right place to put these kinds of asserts, either.

Now, __fentry__ is kind of a special beast; in some ways it is an "exception
implemented as a function call"; on x86 one could even consider using an INT
instruction in order to reduce the NOP footprint in the unarmed case.  Nor is
__fentry__ a C function; it has far more of an exception-like ABI.
*Regardless* of what else we do, I believe __fentry__ ought to
save/disable/restore AC, just like an exception does.

The idea of using s/a gcc plugin/objtool/ for this isn't really a bad idea.
Obviously the general problem is undecidable :) but the enforcement of some
simple, fairly draconian rules ("as tight as possible, but no tighter")
shouldn't be a huge problem.

An actual gcc plugin -- which would probably be quite complex -- could make
gcc itself aware of user space accesses and be able to rearrange them to
minimize STAC/CLAC and avoid kernel-space accesses inside those brackets.

Finally, of course, there is the option of simply outlawing this practice as a
matter of policy and require that all structures be accessed through a limited
set of APIs. As I recall, the number of places where there were
performance-critical regions which could not use the normal accessors are
fairly small (signal frames being the main one.) Doing bulk copy to/from
kernel memory and then accessing them from there would have some performance
cost, but would eliminate the need for this complexity entirely.

	-hpa