linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Message-ID: <CA+55aFyXLm2rd9fKZGYkUQt-F+C=Whra9-5jyvFm3+rG7X0E-g@mail.gmail.com>
Date:	Fri, 27 Mar 2015 13:31:05 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: ia32_sysenter_target does not preserve EFLAGS

On Fri, Mar 27, 2015 at 1:16 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> Does it matter on 32-bit kernels?  There's no swapgs, so IRQs should
> still be safe, and we have a real stack pointer before sysexit.

Fair enough.  On 32-bit, the only worry is the race between "return to
user space" and "something set a thread flag", resulting in delayed
signals and/or higher scheduling latency etc. So on 32-bit, the bug is
much less of an issue, I agree.

So yeah, using sysretl instead of sti+sysexit on 64-bit sounds more
reasonable given the potential worry about sti+sysexit atomicity in
the presense of nmi's.

                      Linus
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