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

From: [email-masked] (Will Deacon)
Date: Mon, 30 Apr 2018 09:31:45 +0100

On Fri, Apr 27, 2018 at 03:55:45PM -0600, Jeffrey Hugo 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 <[email-masked]>
> Reported-by: Jan Glauber <[email-masked]>
> Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> Signed-off-by: Jeffrey Hugo <[email-masked]>
> ---
> 
> 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

Acked-by: Will Deacon <[email-masked]>

Thanks for solving this for all architectures, Jeffrey.

Will