linux-kernel - Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then NMI-safe

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Message-ID: <CA+55aFyAdX_yyFy4XnVQNA+dHc_u7UOqb44t0_iuYKTj1S7M5g@mail.gmail.com>
Date:	Sat, 25 Jul 2015 11:12:29 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, Willy Tarreau <w@....eu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then NMI-safe

On Sat, Jul 25, 2015 at 10:59 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> What if we added something like:
>
> if (regs->ip == ret_after_sti && !user_mode(regs) && (regs->flags &
> X86_EFLAGS_IF)) {
>   regs->ip--;
>   regs->flags &= ~X86_EFLAGS_IF;
> }
>
> to do_nmi, do_machine_check, and do_debug (the latter because kernel
> breakpoints, damnit)?

Hmm. And how would you test it?

Putting an instruction breakpoint on the final 'ret' might do it, I
guess. "mov ss" disables even that (and is documented to disable nmi
too), but maybe it works with 'sti; ret'.

But yes, as long as we'd have some test coverage, that sounds doable.

                Linus
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