Re: [PATCH v7 4/4] arm64/syscalls: Architecture specific pre-usermode check

[Catalin Marinas]

On Mon, Apr 10, 2017 at 09:44:20AM -0700, Thomas Garnier wrote:
> Disable the generic pre-usermode check in favor of an optimized
> implementation. This patch adds specific checks on user-mode return path
> to make it faster and smaller.
> 
> The address limit is checked on each syscall return path to user-mode.
> If it was changed, a generic handler is called to stop the kernel on an
> explicit check.
> 
> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
> Based on next-20170410
> ---
>  arch/arm64/Kconfig        |  1 +
>  arch/arm64/kernel/entry.S | 13 +++++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 9b8fcab7da56..3f9e8e7d9376 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -24,6 +24,7 @@ config ARM64
>  	select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
>  	select ARCH_WANT_FRAME_POINTERS
>  	select ARCH_HAS_UBSAN_SANITIZE_ALL
> +	select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>  	select ARM_AMBA
>  	select ARM_ARCH_TIMER
>  	select ARM_GIC
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 43512d4d7df2..bdd094c7837a 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -744,6 +744,8 @@ ENDPROC(cpu_switch_to)
>  ret_fast_syscall:
>  	disable_irq				// disable interrupts
>  	str	x0, [sp, #S_X0]			// returned x0
> +	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
> +	tbnz	x2, #63, addr_limit_fail
>  	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
>  	and	x2, x1, #_TIF_SYSCALL_WORK
>  	cbnz	x2, ret_fast_syscall_trace
> @@ -771,6 +773,9 @@ work_pending:
>   */
>  ret_to_user:
>  	disable_irq				// disable interrupts
> +	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
> +	tbnz	x2, #63, addr_limit_fail
> +

Nitpick: unnecessary empty line added.

>  	ldr	x1, [tsk, #TSK_TI_FLAGS]
>  	and	x2, x1, #_TIF_WORK_MASK
>  	cbnz	x2, work_pending
> @@ -780,6 +785,14 @@ finish_ret_to_user:
>  ENDPROC(ret_to_user)
>  
>  /*
> + * Address limit was incorrect before returning in user-mode which could be
> + * used to elevate privileges. Call the generic handler to stop the kernel on
> + * the appropriate check. This function does not return.
> + */
> +addr_limit_fail:
> +	bl address_limit_check_failed

You could do a "b address_limit_check_failed" rather than "bl" since we
don't care about returning here (and a nitpick: insert tab between "b"
and "address_limit_check_failed").

With the above:

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>