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 22:04:47 +0200
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 02/18] x86/asm/64: Split the iret-to-user and
 iret-to-kernel paths

On Thu, Oct 26, 2017 at 01:26:34AM -0700, Andy Lutomirski wrote:
> These code paths will diverge soon.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/entry_64.S        | 32 +++++++++++++++++++++++---------
>  arch/x86/entry/entry_64_compat.S |  2 +-
>  arch/x86/kernel/head_64.S        |  2 +-
>  3 files changed, 25 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index afe1f403fa0e..493e5e234d36 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

rstor_regs_ret_user

still sounds pretty ok to me and it is shorter and it is using the
*RSTOR spelling the respective FPU insns use, so should be recognizable. :-)

And the other one could be

rstor_regs_ret_kernel

of course.

>  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 +612,19 @@ 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
> +	testl	$3, CS(%rsp)

testb

ditto for the other spot.

> +	jnz	1f
> +	ud2
> +1:
> +#endif

Why aren't we exploding here unconditionally? I mean, we don't want to
return with RPL != 3 CS for whatever reason...

> +	RESTORE_EXTRA_REGS
> +	RESTORE_C_REGS
> +	REMOVE_PT_GPREGS_FROM_STACK 8
> +	INTERRUPT_RETURN
> +
>  
>  /* Returning to kernel space */
>  retint_kernel:
> @@ -632,11 +644,13 @@ retint_kernel:
>  	 */
>  	TRACE_IRQS_IRETQ
>  
> -/*
> - * At this label, code paths which return to kernel and to user,
> - * which come from interrupts/exception and from syscalls, merge.
> - */
> -GLOBAL(restore_regs_and_iret)
> +GLOBAL(restore_regs_and_return_to_kernel)
> +#ifdef CONFIG_DEBUG_ENTRY
> +	testl	$3, CS(%rsp)
> +	jz	1f
> +	ud2
> +1:
> +#endif

I guess here too, as a sanity-check. TEST and Jcc should be very cheap
compared to everything else jumping-through-hoops we'll be doing :-\

-- 
Regards/Gruss,
    Boris.

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