linux-kernel - Re: [PATCH v1 3/4] arm/syscalls: Specific usage of verify_pre_usermode

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Message-ID: <alpine.LFD.2.20.1703082256480.23532@knanqh.ubzr>
Date:   Wed, 8 Mar 2017 23:05:46 +0100 (CET)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Thomas Garnier <thgarnie@...gle.com>
cc:     Arnd Bergmann <arnd@...db.de>, David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Hansen <dave.hansen@...el.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Petr Mladek <pmladek@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
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Subject: Re: [PATCH v1 3/4] arm/syscalls: Specific usage of
 verify_pre_usermode_state

On Wed, 8 Mar 2017, Thomas Garnier wrote:

> Implement specific usage of verify_pre_usermode_state for user-mode
> returns for arm.
> ---
> Based on next-20170308
> ---
>  arch/arm/Kconfig               | 1 +
>  arch/arm/kernel/entry-common.S | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 0d4e71b42c77..704fd8f197fa 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -12,6 +12,7 @@ config ARM
>  	select ARCH_HAVE_CUSTOM_GPIO_H
>  	select ARCH_HAS_GCOV_PROFILE_ALL
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
> +	select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>  	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
>  	select ARCH_SUPPORTS_ATOMIC_RMW
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index eb5cd77bf1d8..80cfdc7fabde 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -39,6 +39,9 @@
>  ret_fast_syscall:
>   UNWIND(.fnstart	)
>   UNWIND(.cantunwind	)
> +	push	{r0}				@ save returned r0
> +	bl	verify_pre_usermode_state
> +	pop	{r0}				@ restore r0
>  	disable_irq_notrace			@ disable interrupts
>  	ldr	r1, [tsk, #TI_FLAGS]		@ re-check for syscall tracing
>  	tst	r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK

This feature is configurable, right?

Here the branch overhead is imposed even if the feature is configured 
out. You should consider conditionally defining a macro like some other 
features do.

Furthermore I think we still support old toolchains that don't know what 
push and pop mean. You should use the legacy syntax instead.


Nicolas