linux-kernel - Re: [PATCH 02/18] x86/asm/64: Split the iret-to-user and iret-to-kernel paths

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Date:   Fri, 27 Oct 2017 11:05:17 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 02/18] x86/asm/64: Split the iret-to-user and
 iret-to-kernel paths

On 10/26/2017 01:26 AM, Andy Lutomirski wrote:
> +GLOBAL(restore_regs_and_return_to_usermode)
> +#ifdef CONFIG_DEBUG_ENTRY
> +	testl	$3, CS(%rsp)
> +	jnz	1f
> +	ud2

A nit from the mere mortals in the audience: Could we start commenting
or make a constant for the user segment bits in CS?

Also, it would be nice to explain what's going on here.  Maybe:

/*
 * We think we are returning to the kernel.  Check the
 * registers we are about to restore and if we appear to
 * be returning to userspace, do something that will cause
 * a fault and hopefully an oops report.
 */

Otherwise, I really like this change.  It's really hard to figure out
what the context is in the entry assembly in a lot of cases.  It's a
place where code reuse actually makes things harder to follow.