On Sat, Aug 09, 2008 at 09:10:05AM -0700, H. Peter Anvin wrote:
quoted text
> Wolfgang Walter wrote:
> > How could any kernel code use MMX/SSE/FPU when the interrupt case isn't
> > handled?
> 
> I don't think we have ever allowed MMX/SSE/FPU code in interrupt
> handlers.  kernel_fpu_begin()..end() lock out preemption, and so could
> only be interrupted, not preempted.

Yes, fast handlers fall back to slow handlers in the interrupt context
and don't touch FP/SSE and thus avoid the kernel nesting.

hmm, in the padlock interrupt usage scenario(even though it doesn't touch FP/SSE
registers), kernel_fpu_begin/end() will not solve the problem,
as nesting of kernel_fpu_begin() is not ok, as we unconditionally
do stts() in kernel_fpu_end(). So the proposed patch is not ok,
as we end up corrupting first kernel FP usage.

quoted text
> > 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.
> 
> If lazy allocation can cause the RAID code, for example (which executes
> SSE instructions in the kernel, but not at interrupt time) to start
> randomly oopsing, then lazy allocations have to be pulled.

While the lazy allocation is not a big thing and can be pulled(with a
very small patch), this has brought two existing security issues to light
so far. one in lguest code(fixed now) and now in padlock usage. I think even
in 2.6.25, padlock usage can easily can cause the FPU leakage as I mentioned
in another response.

Backing out lazy allocation is not just enough here. Let me think a little
more on this.

thanks,
suresh
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