Re: [PATCH v2] init: Fix false positives in W+X checking

[Kees Cook]

On Fri, Apr 27, 2018 at 2:55 PM, Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx> wrote:
> load_module() creates W+X mappings via __vmalloc_node_range() (from
> layout_and_allocate()->move_module()->module_alloc()) by using
> PAGE_KERNEL_EXEC.  These mappings are later cleaned up via
> "call_rcu_sched(&freeinit->rcu, do_free_init)" from do_init_module().
>
> This is a problem because call_rcu_sched() queues work, which can be run
> after debug_checkwx() is run, resulting in a race condition.  If hit, the
> race results in a nasty splat about insecure W+X mappings, which results
> in a poor user experience as these are not the mappings that
> debug_checkwx() is intended to catch.
>
> This issue is observed on multiple arm64 platforms, and has been
> artificially triggered on an x86 platform.
>
> Address the race by flushing the queued work before running the
> arch-defined mark_rodata_ro() which then calls debug_checkwx().
>
> Reported-by: Timur Tabi <timur@xxxxxxxxxxxxxx>
> Reported-by: Jan Glauber <jan.glauber@xxxxxxxxxxxxxxxxxx>
> Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> Signed-off-by: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>
> v1:
> -was "arm64: mm: Fix false positives in W+X checking" (see [1])
> -moved to common code based on review and confirmation of issue on x86
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-April/573776.html
>
>  init/main.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/init/main.c b/init/main.c
> index b795aa3..499d957 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1034,6 +1034,13 @@ static int __init set_debug_rodata(char *str)
>  static void mark_readonly(void)
>  {
>         if (rodata_enabled) {
> +               /*
> +                * load_module() results in W+X mappings, which are cleaned up
> +                * with call_rcu_sched().  Let's make sure that queued work is
> +                * flushed so that we don't hit false positives looking for
> +                * insecure pages which are W+X.
> +                */
> +               rcu_barrier_sched();
>                 mark_rodata_ro();
>                 rodata_test();
>         } else
> --
> Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
> Qualcomm Technologies, Inc. is a member of the
> Code Aurora Forum, a Linux Foundation Collaborative Project.
>



-- 
Kees Cook
Pixel Security