linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Message-ID: <CAK1hOcPJ+UufD_iGUDOm0wVJiEUzaDRY-XDOQZfCerCNqRmk7Q@mail.gmail.com>
Date:	Sat, 28 Mar 2015 12:01:34 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: ia32_sysenter_target does not preserve EFLAGS

On Sat, Mar 28, 2015 at 1:39 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> What part of "don't leak kernel data" did you have trouble understanding?
>
> IOW, this is a *security* issue. Stop arguing for crazy shit.

We can zero the registers instead of saving/restoring them.
push/pop pair takes 1-2 cycles at best, whereas
on four-issue CPUs "xor reg,reg" has throughput of four
instructions at a time, often even not requiring
an execution unit.
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