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

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Date:   Wed, 8 Mar 2017 21:49:56 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
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>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Brian Gerst <brgerst@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Jan Beulich <JBeulich@...e.com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Laura Abbott <labbott@...hat.com>,
        Andre Przywara <andre.przywara@....com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v1 3/4] arm/syscalls: Specific usage of
 verify_pre_usermode_state

On Wed, Mar 08, 2017 at 01:38:43PM -0800, 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

This really makes the fast exit utterly pointless, and we might as well
rip all that out.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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