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

[Thomas Garnier]

On Mon, Apr 10, 2017 at 1:06 PM, Thomas Garnier <thgarnie at google.com> 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 at google.com>
> ---
> 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
> +
>         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
> +
> +/*
>   * This is how we return from a fork.
>   */
>  ENTRY(ret_from_fork)
> --
> 2.12.2.715.g7642488e1d-goog
>

Ignore this revision, I sent the previous patch by mistake. The next
email is the changed patch.
Sorry about that.


-- 
Thomas