Re: [PATCH v2] mm/vmalloc: widen guard region to defeat ENTER-based stack pivot

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 01, 2026 at 12:58:12AM -0700, H. Peter Anvin wrote:
> On July 1, 2026 12:48:34 AM PDT, Peter Zijlstra <peterz@infradead.org> wrote:
> >On Wed, Jul 01, 2026 at 09:27:47AM +0200, Peter Zijlstra wrote:
> >> On Mon, Jun 29, 2026 at 03:29:50PM -0700, Dave Hansen wrote:
> >> 
> >> > But, really, if ENTER is so evil and nobody uses it, shouldn't we just
> >> > have an MSR bit somewhere to tell the CPU to #UD for it rather than
> >> > playing these stack games?
> >> 
> >> For supervisor mode only, I suppose. We can't ever get rid of userspace
> >> ENTER because legacy I suppose. But we can make sure the kernel is
> >> clean.
> >> 
> >> So yeah, having a knob to make supervisor-ENTER trap would be useful I
> >> suppose.
> >
> >x86_64-defconfig builds clean with the below :-)
> >
> >
> >diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> >index 1b387d5a195b..9e53db863203 100644
> >--- a/tools/objtool/arch/x86/decode.c
> >+++ b/tools/objtool/arch/x86/decode.c
> >@@ -642,6 +642,10 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
> > 
> > 		break;
> > 
> >+	case 0xc8:
> >+		WARN("ENTER instruction at %s:%lx", sec->name, offset);
> >+		break;
> >+
> > 	case 0xc9:
> > 		/*
> > 		 * leave
> 
> The problem is that it being a single byte long it can appear in the middle of another instruction. 

I understand; this was in response to Dave's suggestion to make
(supervisor) ENTER #UD, in which case we'd have to ensure no legit
ENTERs exist.