Re: [RFC PATCH] x86/retpolines: Prevent speculation after RET

[Borislav Petkov <bp@alien8.de>]

On Thu, Feb 18, 2021 at 08:02:31PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 18, 2021 at 07:46:39PM +0100, Borislav Petkov wrote:
> > Both vendors speculate after a near RET in some way:
> > 
> > Intel:
> > 
> > "Unlike near indirect CALL and near indirect JMP, the processor will not
> > speculatively execute the next sequential instruction after a near RET
> > unless that instruction is also the target of a jump or is a target in a
> > branch predictor."
> 
> Right, the way I read that means it's not a problem for us here.

Look at that other thread: the instruction *after* the RET can be
speculatively executed if that instruction is the target of a jump or it
is in a branch predictor.

And yes, the text is confusing and no one from Intel has clarified
definitively yet what that text means exactly.

> Now, if AMD were to say something like: hey, that retpoline is pretty
> awesome, we ought to use that instead of an uconditional LFENCE, then
> sure, but as is, I don't think so.

AMD prefers the LFENCE instead of the ratpoline sequence.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette