linux-kernel - Re: [RFC v3 1/2] x86/xen: add xen_is_preemptible

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:	Wed, 21 Jan 2015 19:07:36 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jan Beulich <JBeulich@...e.com>
Subject: Re: [RFC v3 1/2] x86/xen: add xen_is_preemptible_hypercall()

On Wed, Jan 21, 2015 at 6:17 PM, Luis R. Rodriguez
<mcgrof@...not-panic.com> wrote:
> From: "Luis R. Rodriguez" <mcgrof@...e.com>
>
> On kernels with voluntary or no preemption we can run
> into situations where a hypercall issued through userspace
> will linger around as it addresses sub-operatiosn in kernel
> context (multicalls). Such operations can trigger soft lockup
> detection.
>
> We want to address a way to let the kernel voluntarily preempt
> such calls even on non preempt kernels, to address this we first
> need to distinguish which hypercalls fall under this category.
> This implements xen_is_preemptible_hypercall() which lets us do
> just that by adding a secondary hypercall page, calls made via
> the new page may be preempted.
>
> Andrew had originally submitted a version of this work [0].
>
> [0] http://lists.xen.org/archives/html/xen-devel/2014-02/msg01056.html
>
> Based on original work by: Andrew Cooper <andrew.cooper3@...rix.com>
>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: David Vrabel <david.vrabel@...rix.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Cc: Jan Beulich <JBeulich@...e.com>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
> ---
>  arch/x86/include/asm/xen/hypercall.h | 20 ++++++++++++++++++++
>  arch/x86/xen/enlighten.c             |  7 +++++++
>  arch/x86/xen/xen-head.S              | 18 +++++++++++++++++-
>  3 files changed, 44 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
> index ca08a27..221008e 100644
> --- a/arch/x86/include/asm/xen/hypercall.h
> +++ b/arch/x86/include/asm/xen/hypercall.h
> @@ -84,6 +84,22 @@
>
>  extern struct { char _entry[32]; } hypercall_page[];
>
> +#ifndef CONFIG_PREEMPT
> +extern struct { char _entry[32]; } preemptible_hypercall_page[];

A comment somewhere explaining why only non-preemptible kernels have
preemptible hypercalls might be friendly to some future reader. :)

> +
> +static inline bool xen_is_preemptible_hypercall(struct pt_regs *regs)
> +{
> +       return !user_mode_vm(regs) &&
> +               regs->ip >= (unsigned long)preemptible_hypercall_page &&
> +               regs->ip < (unsigned long)preemptible_hypercall_page + PAGE_SIZE;
> +}

This makes it seem like the page is indeed one page long, but I don't
see what actually allocates a whole page for it.  What am I missing?

--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/