Messages in this thread | | | Date | Fri, 23 Jan 2015 13:07:23 -0800 | From | "H. Peter Anvin" <> | Subject | Re: question about save_xstate_sig() - WHY DOES THIS WORK? |
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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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