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

[Xin Li]

> > Something like so... except this is broken. Its reporting spurious
> > interrupts on vector 0x00, so something is buggered passing that vector
> > along.

I'm a bit late to the party :)

Peter kind of got what I had in the FRED patch set v8 or earlier:

https://lore.kernel.org/lkml/20230410081438.1750-34-xin3.li@xxxxxxxxx/


> 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.

+1

> 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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> ---
>   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

Neat!

I ever had a plan to do this with "sub $0x8,%rsp; iret;" for non-FRED
case.  But obviously doing nothing here is the best.

>   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

This is a smart change.

When !X86_FEATURE_FRED, the FRED stack frame set up for ERETS is
implicitly skipped by leave.  Maybe add a comment to explain LEAVE works
for both cases?