netdev - Re: Kernel oops with 2.6.26, padlock and ipsec: probably problem with fpu state changes

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Date:	Sun, 10 Aug 2008 00:59:10 +0200
From:	Wolfgang Walter <wolfgang.walter@...m.de>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	"viro@...IV.linux.org.uk" <viro@...iv.linux.org.uk>,
	"vegard.nossum@...il.com" <vegard.nossum@...il.com>
Subject: Re: Kernel oops with 2.6.26, padlock and ipsec: probably problem with fpu state changes

On Saturday 09 August 2008, Suresh Siddha wrote:
> On Sat, Aug 09, 2008 at 11:52:24AM -0700, Siddha, Suresh B wrote:
> > Backing out lazy allocation is not just enough here. Let me think a little
> > more on this.
> 
> Can we have something like irq_ts_save() and irq_ts_restore(), which will
> do something like:
> 
> int irq_ts_save()
> {
> 	if (!in_interrupt())
> 		return 0;
> 
> 	if (read_cr0() & X86_CR0_TS) {
> 		clts();
> 		return 1;
> 	}
> 	return 0;
> }
> 
> void irq_ts_restore(int TS_state)
> {
> 	if (!in_interrupt())
> 		return 0;
> 
> 	if (TS_state)
> 		stts();
> }
> 
> and use this around padlock usage. Taking a spurious DNA fault in the 
process
> context(even inside the kernel) should be ok. Main issue is with the 
interrupt
> context and we can prevent the DNA fault in the irq context using above.
> 
> Either above, or we have to remove the lazy fpu allocation and make the
> below code in kernel_fpu_begin() atomic by disabling interrupts(to fix
> the security hole with padlock usage)
> 
> kernel_fpu_begin:
> 	...
> 
> 	local_irq_disable();
> 
>         if (me->status & TS_USEDFPU)
>                 __save_init_fpu(me->task);
>         else
>                 clts(); 
> 
> 	local_irq_enable();
> 	...
> 
> 

The first solution - if it works and padlock is the only which has problem 
with it - seems to be a good fix for 2.6.26. If it works I can't say as I'm 
not familiar enough with these things. But I'll happily test it :-).

The second would be a little bit intrusive, wouldn't it? Most machines don't 
have padlock, and therfore don't need this change but nevertheless may be 
affected (i.e. they use MMX for memcpy or MMX/SSE with raid6) and now get a 
different behaviour. Don't know how expensive such a local_irq_enable/disable 
would be.


Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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