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

[Wolfgang Walter]

On Saturday 09 August 2008, Herbert Xu wrote:
> On Fri, Aug 08, 2008 at 04:11:21PM -0700, Suresh Siddha wrote:
> >
> > With out the recent dynamic fpu allocation changes, while we don't see 
oops,
> > there is a possible race still present in older kernels(for example,
> > while kernel is using kernel_fpu_begin() in some optimized clear/copy
> > page and an interrupt/softirq happens which uses these padlock
> > instructions generating DNA fault).
> 
> No this wasn't a problem because kernel_fpu_begin clears TS and
> therefore we don't get faults on SSE instructions.
> 
> However, with your patch it will become a problem due to the
> fact that it wasn't designed to be nested.
> 


I don't exactly understand this. You think that

kernel_fpu_begin();
XCRYPT....
kernel_fpu_end();

is a problem and wasn't before?

Say we have a software crypt-alg which uses optimized memcpy implemented with 
SSE instructions. These are protected with kernel_fpu_begin(); 
kernel_fpu_end();

So we have also code

kernel_fpu_begin();
SSE....
kernel_fpu_end();

in crypto called under same circumstances.

If XCRYPT may be interrupted and the interrupt code again uses this optimized 
memcpy implementation and so nesting kernel_fpu_begin then why should this 
not happen with the other alg.

How could any kernel code use MMX/SSE/FPU when the interrupt case isn't 
handled?


Or is your argument that its lazy allocation itself is the problem: this 
nesting could always happen and was a bug but only with lazy allocation it is 
dangerous (as it may cause a spurious math fault in the race window).

If this were right than any kernel code executing SSE may trigger now a oops 
in __switch_to() under some special circumstances.


Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
Leiter EDV
Leopoldstraße 15
80802 München
Tel: +49 89 38196-276
Fax: +49 89 38196-144
wolfgang.walter@xxxxxxx
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