linux-kernel - Re: [PATCH] paravirt.h

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: <44EC140E.5050703@vmware.com>
Date:	Wed, 23 Aug 2006 01:38:38 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	virtualization@...ts.osdl.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] paravirt.h

Andi Kleen wrote:
>  
>   
>> Well, I don't think anything is sufficient for a preemptible kernel.  I 
>> think that's just plain not going to work.  You could have a kernel 
>> thread that got preempted in a paravirt-op patch point, and making all 
>> the patch points non-preempt is probably a non-starter (either +12 bytes 
>> each or no native inlining). 
>>     
>
> stop machine deals with preemption.  If it didn't it would be unusable
> for the purposes the kernel uses it right now (cpu hotplug, module unloading etc.)
>   

Yes, but it can't move pre-empted threads out of a particularly 
dangerous EIP (like a piece of code we're about to patch over).  Or 
perhaps I am misunderstanding how it deals with preemption, and what it 
really does is make sure all threads are in userspace or sleep state...  
which in that case is perfectly fine.

> and machine checks. debug traps -- i assume you mean kernel debuggers -- 
> sounds like something that cannot be really controlled though.
>
> How do you control a debugger from the debugee?
>
> I don't think NMI/MCEs are a problem though because NMIs (at least oprofile/nmi watchdog) 
> and MCEs all just have global state that can be changed on a single CPU.
>   

But with paravirt-ops, that global state may include local CPU state, in 
which paravirt-ops is intimately involved.  So they could interrupt in 
the middle of the patching code, then attempt a paravirt_ops call, which 
is in an undefined state until the patching is complete.  And I would 
highly expect the debugger to  mess with debug registers, which is a 
paravirt op.  NMIs can do plenty of dangerous things to local state as 
well - reading and writing MSRs or performance counters I would imagine 
to be quite useful.

Zach
-
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/