linux-kernel - Re: [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 30 Sep 2014 15:30:02 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Sebastian Lackner <sebastian@...-team.de>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>
Subject: Re: [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch_to

On Tue, Sep 30, 2014 at 3:21 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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.

We could do something hideous:

Don't filter NT in sysexit or on context switch.  Instead, handle it
in bad_iret.

Up side: all common cases are maximally fast.

Down side: Ugly.  And malicious processes can leak NT, causing return
to a different process to fault, thereby adding a thousand or two
cycles (or possibly a lot more if the fault hits in the middle of
espfix64.  Egads.)

This is not intended to be a serious suggestion...

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/