Re: [PATCH] x86/entry/32: Harden return-to-user

[Dave Hansen]

On 10/27/26 14:52, Andrey Zakharov wrote:
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -952,6 +952,14 @@ restore_all_switch_stack:
>  
>  	/* Restore user state */
>  	RESTORE_REGS pop=4			# skip orig_eax/error_code
> +
> +	/* Assert that the IRET frame returns to user mode or vm86. */
> +	testb	$USER_SEGMENT_RPL_MASK, 4(%esp)
> +	jnz	.Lreturn_to_user_ok
> +	testl	$X86_EFLAGS_VM, 8(%esp)
> +	jnz	.Lreturn_to_user_ok
> +	ud2
> +.Lreturn_to_user_ok:

I'm curious who cares.

Is anyone who is actually still running a 32-bit kernel concerned about
this kind of hardening?

If no one cares, then I'd rather pass on the complexity, even four
instructions of it.