Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 09, 2018 at 06:26:47PM -0800, Tim Chen wrote:
> Set IBRS upon kernel entrance via syscall and interrupts. Clear it
> upon exit.  IBRS protects against unsafe indirect branching predictions
> in the kernel.
> 
> The NMI interrupt save/restore of IBRS state was based on Andrea
> Arcangeli's implementation.
> Here's an explanation by Dave Hansen on why we save IBRS state for NMI.
> 
> The normal interrupt code uses the 'error_entry' path which uses the
> Code Segment (CS) of the instruction that was interrupted to tell
> whether it interrupted the kernel or userspace and thus has to switch
> IBRS, or leave it alone.
> 
> The NMI code is different.  It uses 'paranoid_entry' because it can
> interrupt the kernel while it is running with a userspace IBRS (and %GS
> and CR3) value, but has a kernel CS.  If we used the same approach as
> the normal interrupt code, we might do the following;
> 
> 	SYSENTER_entry
> <-------------- NMI HERE
> 	IBRS=1
> 		do_something()
> 	IBRS=0
> 	SYSRET
> 
> The NMI code might notice that we are running in the kernel and decide
> that it is OK to skip the IBRS=1.  This would leave it running
> unprotected with IBRS=0, which is bad.
> 
> However, if we unconditionally set IBRS=1, in the NMI, we might get the
> following case:
> 
> 	SYSENTER_entry
> 	IBRS=1
> 		do_something()
> 	IBRS=0
> <-------------- NMI HERE (set IBRS=1)
> 	SYSRET
> 
> and we would return to userspace with IBRS=1.  Userspace would run
> slowly until we entered and exited the kernel again.
> 
> Instead of those two approaches, we chose a third one where we simply
> save the IBRS value in a scratch register (%r13) and then restore that
> value, verbatim.
> 

What this Changelog fails to address is _WHY_ we need this. What does
this provide that retpoline does not.