linux-kernel - Re: [PATCH v8 1/2] x86/kexec: add a new atomic notifier list for kdump

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]

Message-ID: <20121126175327.GG12969@redhat.com>
Date:	Mon, 26 Nov 2012 19:53:27 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v8 1/2] x86/kexec: add a new atomic notifier list for
 kdump

On Mon, Nov 26, 2012 at 11:43:10AM -0600, Eric W. Biederman wrote:
> Gleb Natapov <gleb@...hat.com> writes:
> 
> > On Mon, Nov 26, 2012 at 09:08:54AM -0600, Eric W. Biederman wrote:
> >> Zhang Yanfei <zhangyanfei@...fujitsu.com> writes:
> >> 
> >> > This patch adds an atomic notifier list named crash_notifier_list.
> >> > Currently, when loading kvm-intel module, a notifier will be registered
> >> > in the list to enable vmcss loaded on all cpus to be VMCLEAR'd if
> >> > needed.
> >> 
> >> crash_notifier_list ick gag please no.  Effectively this makes the kexec
> >> on panic code path undebuggable.
> >> 
> >> Instead we need to use direct function calls to whatever you are doing.
> >> 
> > The code walks linked list in kvm-intel module and calls vmclear on
> > whatever it finds there. Since the function have to resides in kvm-intel
> > module it cannot be called directly. Is callback pointer that is set
> > by kvm-intel more acceptable?
> 
> Yes a specific callback function is more acceptable.  Looking a little
> deeper vmclear_local_loaded_vmcss is not particularly acceptable. It is
> doing a lot of work that is unnecessary to save the virtual registers
> on the kexec on panic path.
> 
What work are you referring to in particular that may not be acceptable?

> In fact I wonder if it might not just be easier to call vmcs_clear to a
> fixed per cpu buffer.
> 
There may be more than one vmcs loaded on a cpu, hence the list.

> Performing list walking in interrupt context without locking in
> vmclear_local_loaded vmcss looks a bit scary.  Not that locking would
> make it any better, as locking would simply add one more way to deadlock
> the system.  Only an rcu list walk is at all safe.  A list walk that
> modifies the list as vmclear_local_loaded_vmcss does is definitely not safe.
> 
The list vmclear_local_loaded walks is per cpu. Zhang's kvm patch
disables kexec callback while list is modified.

--
			Gleb.
--
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/