Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig()

Borislav Petkov
2020-02-28T12:17:34Z

On Thu, Feb 27, 2020 at 02:52:12PM -0800, Yu-cheng Yu wrote:
> > So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
> > this?
> >
> > Why don't you simply do:
> >
> > set_thread_flag(TIF_NEED_FPU_LOAD);
> > fpregs_lock();
> > if (xfeatures_mask_supervisor())
> > copy_xregs_to_kernel(&fpu->state.xsave);
> > fpregs_unlock();
>
> If TIF_NEED_FPU_LOAD is set, then xstates are already in the xsave buffer.
> We can skip saving them again.

Ok, then pls use test_and_set_thread_flag().

Also, in talking to Sebastian about this on IRC, he raised a valid
concern: if we are going to save supervisor states here, then
copy_xregs_to_kernel() should better save *only* supervisor states
because we're not interested in the user states - they're going to be
overwritten with the states from the stack.

So copy_xregs_to_kernel() needs to learn about a second parameter called
@mask like copy_kernel_to_xregs().

Thx.

--
Regards/Gruss,
Boris.