Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

["H. Peter Anvin" <hpa@zytor.com>]

On May 1, 2025 11:30:18 AM PDT, Sean Christopherson <seanjc@google.com> wrote:
>On Thu, May 01, 2025, Peter Zijlstra wrote:
>> On Thu, May 01, 2025 at 12:30:38PM +0200, Peter Zijlstra wrote:
>> > On Wed, Apr 30, 2025 at 09:06:00PM +0200, Peter Zijlstra wrote:
>> > > On Wed, Apr 30, 2025 at 07:24:15AM -0700, H. Peter Anvin wrote:
>> > > 
>> > > > >KVM has another; the VMX interrupt injection stuff calls the IDT handler
>> > > > >directly.  Is there an alternative? Can we keep a table of Linux functions
>> > > > >slighly higher up the call stack (asm_\cfunc ?) and add CFI to those?
>> > > 
>> > > > We do have a table of handlers higher up in the stack in the form of
>> > > > the dispatch tables for FRED. They don't in general even need the
>> > > > assembly entry stubs, either.
>> > > 
>> > > Oh, right. I'll go have a look at those.
>> > 
>> > Right, so perhaps the easiest way around this is to setup the FRED entry
>> > tables unconditionally, have VMX mandate CONFIG_FRED and then have it
>> > always use the FRED entry points.
>> > 
>> > Let me see how ugly that gets.
>> 
>> Something like so... except this is broken. Its reporting spurious
>> interrupts on vector 0x00, so something is buggered passing that vector
>> along.
>
>Uh, aren't you making this way more complex than it needs to be?  IIUC, KVM never
>uses the FRED hardware entry points, i.e. the FRED entry tables don't need to be
>in place because they'll never be used.  The only bits of code KVM needs is the
>__fred_entry_from_kvm() glue.
>
>Lightly tested, but this combo works for IRQs and NMIs on non-FRED hardware.
>
>--
>From 664468143109ab7c525c0babeba62195fa4c657e Mon Sep 17 00:00:00 2001
>From: Sean Christopherson <seanjc@google.com>
>Date: Thu, 1 May 2025 11:20:29 -0700
>Subject: [PATCH 1/2] x86/fred: Play nice with invoking
> asm_fred_entry_from_kvm() on non-FRED hardware
>
>Modify asm_fred_entry_from_kvm() to allow it to be invoked by KVM even
>when FRED isn't fully enabled, e.g. when running with CONFIG_X86_FRED=y
>on non-FRED hardware.  This will allow forcing KVM to always use the FRED
>entry points for 64-bit kernels, which in turn will eliminate a rather
>gross non-CFI indirect call that KVM uses to trampoline IRQs by doing IDT
>lookups.
>
>When FRED isn't enabled, simply skip ERETS and restore RBP and RSP from
>the stack frame prior to doing a "regular" RET back to KVM (in quotes
>because of all the RET mitigation horrors).
>
>Signed-off-by: Sean Christopherson <seanjc@google.com>
>---
> arch/x86/entry/entry_64_fred.S | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
>index 29c5c32c16c3..7aff2f0a285f 100644
>--- a/arch/x86/entry/entry_64_fred.S
>+++ b/arch/x86/entry/entry_64_fred.S
>@@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> 	movq %rsp, %rdi				/* %rdi -> pt_regs */
> 	call __fred_entry_from_kvm		/* Call the C entry point */
> 	POP_REGS
>-	ERETS
>+
>+	ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED
> 1:
> 	/*
> 	 * Objtool doesn't understand what ERETS does, this hint tells it that
>@@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> 	 * isn't strictly needed, but it's the simplest form.
> 	 */
> 	UNWIND_HINT_RESTORE
>-	pop %rbp
>+	leave
> 	RET
> 
> SYM_FUNC_END(asm_fred_entry_from_kvm)
>
>base-commit: 45eb29140e68ffe8e93a5471006858a018480a45

Ok maybe I'm being dense, but what is left other than simply calling __fred_entry_from_kvm() as a normal C function? 

I'm on the go so there might be something in the code I'm missing, but on the surface...?