linux-kernel - Re: [PATCH v2 02/20] x86/asm/64: Split the iret-to-user and iret-to-kernel paths

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Date:   Thu, 2 Nov 2017 11:50:18 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 02/20] x86/asm/64: Split the iret-to-user and
 iret-to-kernel paths

On Thu, Nov 02, 2017 at 12:58:59AM -0700, Andy Lutomirski wrote:
> These code paths will diverge soon.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/entry_64.S        | 34 +++++++++++++++++++++++++---------
>  arch/x86/entry/entry_64_compat.S |  2 +-
>  arch/x86/kernel/head_64.S        |  2 +-
>  3 files changed, 27 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index afe1f403fa0e..07fe816f0d28 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -321,7 +321,7 @@ syscall_return_via_sysret:
>  
>  opportunistic_sysret_failed:
>  	SWAPGS
> -	jmp	restore_regs_and_iret
> +	jmp	restore_regs_and_return_to_usermode
>  END(entry_SYSCALL_64)
>  
>  ENTRY(stub_ptregs_64)
> @@ -423,7 +423,7 @@ ENTRY(ret_from_fork)
>  	call	syscall_return_slowpath	/* returns with IRQs disabled */
>  	TRACE_IRQS_ON			/* user mode is traced as IRQS on */
>  	SWAPGS
> -	jmp	restore_regs_and_iret
> +	jmp	restore_regs_and_return_to_usermode
>  
>  1:
>  	/* kernel thread */
> @@ -612,7 +612,20 @@ GLOBAL(retint_user)
>  	call	prepare_exit_to_usermode
>  	TRACE_IRQS_IRETQ
>  	SWAPGS
> -	jmp	restore_regs_and_iret
> +
> +GLOBAL(restore_regs_and_return_to_usermode)
> +#ifdef CONFIG_DEBUG_ENTRY
> +	/* Assert that pt_regs indicates user mode. */
> +	testl	$3, CS(%rsp)
> +	jnz	1f
> +	ud2
> +1:
> +#endif

Here's me arguing in v2:

If these paths are slow and adding a TEST and a Jcc would give us the
additional sanity-checking, then I don't see any downside to it.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--