Re: [PATCH v18 21/25] x86/cet/shstk: Handle signals for shadow stack

[Dave Hansen]

On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
> To deliver a signal, create a shadow stack restore token and put a restore
> token and the signal restorer address on the shadow stack.  For sigreturn,
> verify the token and restore the shadow stack pointer.
> 
> Introduce WRUSS, which is a kernel-mode instruction but writes directly to
> user shadow stack.  It is used to construct the user signal stack as
> described above.
> 
> Introduce a signal context extension struct 'sc_ext', which is used to save
> shadow stack restore token address and WAIT_ENDBR status.  WAIT_ENDBR will
> be introduced later in the Indirect Branch Tracking (IBT) series, but add
> that into sc_ext now to keep the struct stable in case the IBT series is
> applied later.

This changelog needs some work.  It's got a lot of "what" and not enough
"why".

Why do we need a token?
What function does it serve?
What does it protect against?
Why do we need a signal context extension?