Re: [PATCH] x86/nospec: Unwreck the RSB stuffing

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Aug 16, 2022 at 02:28:36PM +0200, Peter Zijlstra wrote:
> 
> Replying here, because obviously there's no actual posting of this
> patch... :/

{sigh} True :(

> 
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -118,13 +118,28 @@
> >  #endif
> >  .endm
> >  
> > +.macro ISSUE_UNBALANCED_RET_GUARD
> > +	ANNOTATE_INTRA_FUNCTION_CALL
> > +	call .Lunbalanced_ret_guard_\@
> > +	int3
> > +.Lunbalanced_ret_guard_\@:
> > +	add $(BITS_PER_LONG/8), %_ASM_SP
> > +	lfence
> > +.endm
> > +
> >   /*
> >    * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
> >    * monstrosity above, manually.
> >    */
> > -.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
> > +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2
> > +.ifb \ftr2
> >  	ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
> > +.else
> > +	ALTERNATIVE_2 "jmp .Lskip_rsb_\@", "", \ftr, "jmp .Lunbalanced_\@", \ftr2
> > +.endif
> >  	__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
> > +.Lunbalanced_\@:
> > +	ISSUE_UNBALANCED_RET_GUARD
> >  .Lskip_rsb_\@:
> >  .endm
> 
> (/me deletes all the swear words and starts over)
> 
> This must absolutely be the most horrible patch you could come up with,
> no? I suppose that's the price of me taking PTO :-(
> 
> Could you please test this; I've only compiled it.
> 
> ---
> Subject: x86/nospec: Unwreck the RSB stuffing
> 
> Commit 2b1299322016 ("x86/speculation: Add RSB VM Exit protections")
> made a right mess of the RSB stuffing, rewrite the whole thing to not
> suck.
> 
> Thanks to Andrew for the enlightening comment about Post-Barrier RSB
> things so we can make this code less magical.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

I need an Intel person to test this as I have no idea how to do so as
this is an issue in Linus's tree.

thanks,

greg k-h