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

Wolfgang Walter
2008-08-09T22:59:24Z

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