Re: [PATCH] x86/bugs: Allow STIBP with IBRS

Josh Poimboeuf
2023-02-20T19:19:46Z

On Mon, Feb 20, 2023 at 11:04:56AM -0800, KP Singh wrote:
> On Mon, Feb 20, 2023 at 11:00 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> >
> > On Mon, Feb 20, 2023 at 10:33:56AM -0800, KP Singh wrote:
> > > static char *stibp_state(void)
> > > {
> > > - if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
> > > + if (!spectre_v2_user_needs_stibp(spectre_v2_enabled))
> > > return "";
> > >
> > > switch (spectre_v2_user_stibp) {
> > >
> > > Also Josh, is it okay for us to have a discussion and have me write
> > > the patch as a v2? Your current patch does not even credit me at all.
> > > Seems a bit unfair, but I don't really care. I was going to rev up the
> > > patch with your suggestions.
> >
> > Well, frankly the patch needed a complete rewrite. The patch
> > description was unclear about what the problem is and what's being
>
> Josh, this is a complex issue, we are figuring it out together on the
> list. It's complex, that's why folks got it wrong in the first place.
> Calling the patch obtuse and unclear is unfair!
>
> > fixed. The code was obtuse and the comments didn't help. I could tell
> > by the other replies that I wasn't the only one confused.
>
> The patch you sent is not clear either, it implicitly ties in STIBP
> with eIBRS. There is no explanation anywhere that IBRS just means
> KERNEL_IBRS.

Ok, so something like this on top?

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b97c0d28e573..fb3079445700 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1201,6 +1201,10 @@ spectre_v2_user_select_mitigation(void)
/*
* If no STIBP, enhanced IBRS is enabled, or SMT impossible,
* STIBP is not required.
+ *
+ * For legacy IBRS, STIBP may still be needed because IBRS is only
+ * enabled in kernel space, so user space isn't protected from indirect
+ * branch prediction attacks from a sibling CPU thread.
*/
if (!boot_cpu_has(X86_FEATURE_STIBP) ||
!smt_possible ||