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From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: X86 ML <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Sebastian Lackner <sebastian@fds-team.de>,
	Anish Bhatt <anish@chelsio.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Chuck Ebbert <cebbert.lkml@gmail.com>
Subject: Re: [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch_to
Date: Wed, 1 Oct 2014 00:21:55 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.11.1410010019290.4455@nanos> (raw)
In-Reply-To: <cover.1412105369.git.luto@amacapital.net>

On Tue, 30 Sep 2014, Andy Lutomirski wrote:
> Anish Bhatt noticed that user programs can set RFLAGS.NT before
> syscall or sysenter, and the kernel entry code doesn't filter out
> NT.  This causes kernel C code and, depending on thread flags, the
> exit slow path to run with NT set.
> 
> The former is a little bit scary (imagine calling into EFI with NT
> set), and the latter will fail with #GP and send a spurious SIGSEGV.
> 
> One answer would be "don't do that".  But the kernel can do better
> here.
> 
> These patches, which I'm not completely thrilled by, filter NT on
> all kernel entries.  For syscall (both bitnesses), this is free.
> For sysenter, it costs 15 cycles or so.  As a consolation prize, we
> can speed up context switches by avoiding saving and restoring flags.

That's a nice reason not to do any of the other ugly variants.

Thanks,

	tglx

  parent reply	other threads:[~2014-09-30 22:22 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-30 19:40 Andy Lutomirski
2014-09-30 19:40 ` [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace Andy Lutomirski
2014-09-30 21:39   ` Sebastian Lackner
2014-09-30 21:45     ` Andy Lutomirski
2014-09-30 22:23       ` Sebastian Lackner
2014-09-30 22:27       ` Thomas Gleixner
2014-09-30 22:33         ` Andy Lutomirski
2014-09-30 23:21           ` Thomas Gleixner
2014-10-01 17:50             ` H. Peter Anvin
2014-10-01 17:53               ` H. Peter Anvin
2014-09-30 22:42         ` H. Peter Anvin
2014-10-01  0:27   ` Chuck Ebbert
2014-10-01  0:38     ` Andy Lutomirski
2014-09-30 19:40 ` [PATCH 2/2] x86_64: Don't save flags on context switch Andy Lutomirski
2014-09-30 22:21 ` Thomas Gleixner [this message]
2014-09-30 22:30   ` [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch_to Andy Lutomirski

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