linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Message-ID: <CALCETrVDnXzkk=jryh4X4gO0TU9ziAK6Ecp9-OXwwXcTFucu5A@mail.gmail.com>
Date:	Fri, 27 Mar 2015 14:11:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 1:31 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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.

Right, except for one nasty case: KVM user return notifiers.  It's
possible we'd re-enter user mode with some MSRs set wrong.  Yuck.

--Andy

>
> 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



-- 
Andy Lutomirski
AMA Capital Management, LLC
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