Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

[Yu, Yu-cheng]

On 8/27/2021 11:21 AM, Borislav Petkov wrote:
> On Fri, Aug 27, 2021 at 11:10:31AM -0700, Yu, Yu-cheng wrote:
> > Because on context switches the whole xstates are switched together,
> > we need to make sure all are in registers.
>
> There's context switch code which does that already.
>
> Why would shstk_setup() be responsible for switching the whole extended
> states buffer instead of only the shadow stack stuff only?

Right now, the kernel does lazy restore, and it waits until right before 
a task goes back to ring-3 to restore xstates.  If a task needs to write 
to any xstate registers before that (e.g. for signals), it restores the 
whole xstates first and clears TIF_NEED_FPU_LOAD, which will prevent 
xstates being restored again later.