linux-kernel - Re: [PATCH 03/18] x86/asm/64: Move SWAPGS into the common iret-to-usermode path

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Message-ID: <07bbde42-91da-5f7e-d185-d21f6abc9323@intel.com>
Date:   Fri, 27 Oct 2017 11:08:28 -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 03/18] x86/asm/64: Move SWAPGS into the common
 iret-to-usermode path

On 10/26/2017 01:26 AM, Andy Lutomirski wrote:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 493e5e234d36..1909a4e42b81 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -254,7 +254,7 @@ return_from_SYSCALL_64:
>  	movq	RCX(%rsp), %rcx
>  	movq	RIP(%rsp), %r11
>  	cmpq	%rcx, %r11			/* RCX == RIP */
> -	jne	opportunistic_sysret_failed
> +	jne	swapgs_restore_regs_and_return_to_usermode

Could we just leave the "opportunistic_sysret_failed" label and put it
at the same location as "swapgs_restore_regs_and_return_to_usermode".
It's kinda nice to have the failure paths spelled out.

Just reading this, I have no idea if "RCX == RIP" is good or bad and
whether we're proceeding with the sysret or giving up on it.