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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

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.

-- 
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