Re: question about save_xstate_sig() - WHY DOES THIS WORK?

[H. Peter Anvin]

On 01/23/2015 11:34 AM, Rik van Riel wrote:
> While working on a patch series to defer FPU state loading until
> kernel -> user space transition, and be more lazy with FPU state
> while in the kernel, I came across this code in save_xstate_sig().
> 
> Not only is this broken with my new code, but it looks like it may
> be broken with the current code, too...
> 
> Specifically, save_user_xstate() may page fault and sleep. After
> returning from the page fault, there is no guarantee that the
> FPU state will be restored into the CPU, when the system is not
> running with eager fpu mode.
> 
> In that case, what prevents us from saving random FPU register state
> to the user's stack frame?  Potentially state containing data from
> other programs...
> 

If the FPU state is not current, we'll have CR0.TS = 1 and the XSAVE
will cause an #NM exception, which will cause the FPU state to be
swapped in.

	-hpa


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