From: Andy Lutomirski <luto@amacapital.net>
To: Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
Cc: 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>,
Andy Lutomirski <luto@amacapital.net>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace
Date: Wed, 1 Oct 2014 12:49:56 -0700 [thread overview]
Message-ID: <CALCETrUPqT3WR9-B4mjS791i+nK0ApDyqf9dr0cB_vSRAbgDGQ@mail.gmail.com> (raw)
In-Reply-To: <395749a5d39a29bd3e4b35899cf3a3c1340e5595.1412189265.git.luto@amacapital.net>
On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> The NT flag doesn't do anything in long mode other than causing IRET
> to #GP. Oddly, CPL3 code can still set NT using popf.
>
[...]
> +
> + /*
> + * Sysenter doesn't filter flags, so we need to clear NT
> + * ourselves. To save a few cycles, we can check whether
> + * NT was set instead of doing an unconditional popfq.
> + */
> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */
> + jnz sysenter_fix_flags
> +sysenter_flags_fixed:
> +
Because this thread hasn't gone on long enough:
Do we need to clear IOPL here, too? With patch 2 applied, an IOPL !=
0 program can leak IOPL into another task. It should be cleared on
iret, sysexit (via popf) and sysret (directly), so this shouldn't
matter. Am I missing something?
Adding IOPL to the test will add no overhead for non-iopl-using tasks,
but it will slighly slow down 32-bit tasks that use iopl.
--Andy
next prev parent reply other threads:[~2014-10-01 19:50 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-01 18:49 [PATCH v4 0/2] x86_64,entry: Clear NT on entry and speed up switch_to Andy Lutomirski
2014-10-01 18:49 ` [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace Andy Lutomirski
2014-10-01 19:49 ` Andy Lutomirski [this message]
2014-10-02 15:36 ` H. Peter Anvin
2014-10-06 16:42 ` H. Peter Anvin
2014-10-06 18:07 ` [tip:x86/urgent] x86_64, entry: " tip-bot for Andy Lutomirski
2014-10-01 18:49 ` [PATCH v4 2/2] x86_64: Don't save flags on context switch Andy Lutomirski
2014-10-06 16:39 ` [PATCH v4 0/2] x86_64,entry: Clear NT on entry and speed up switch_to Andy Lutomirski
2014-10-06 16:41 ` H. Peter Anvin
2014-10-06 16:45 ` Andy Lutomirski
2014-10-06 16:57 ` H. Peter Anvin
2014-11-01 0:20 ` Rusty Russell
2014-11-01 1:00 ` Andy Lutomirski
2014-11-01 1:08 ` [PATCH] x86_64, entry: Fix out of bounds read on sysenter Andy Lutomirski
2014-11-01 2:28 ` Rusty Russell
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