Re: [PATCH v3] init: Fix false positives in W+X checking

[Laura Abbott]

On 04/30/2018 08:59 AM, 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 <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>
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Acked-by: Will Deacon <will.deacon@xxxxxxx>
> ---

Acked-by: Laura Abbott <labbott@xxxxxxxxxx>

If you don't have a tree for this to go through, I might suggest having
Kees take it.

> v3:
> -added comment to module code to establish matching connection
> -added acks by Kees, Ingo, and Will
>
> v2:
> -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 +++++++
>   kernel/module.c | 5 +++++
>   2 files changed, 12 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
> diff --git a/kernel/module.c b/kernel/module.c
> index a6e43a5..7b71c3c 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3516,6 +3516,11 @@ static noinline int do_init_module(struct module *mod)
>   	 * walking this with preempt disabled.  In all the failure paths, we
>   	 * call synchronize_sched(), but we don't want to slow down the success
>   	 * path, so use actual RCU here.
> +	 * Note that module_alloc() on most architectures creates W+X page
> +	 * mappings which won't be cleaned up until do_free_init() runs.  Any
> +	 * code such as mark_rodata_ro() which depends on those mappings to
> +	 * be cleaned up needs to sync with the queued work - ie
> +	 * rcu_barrier_sched()
>   	 */
>   	call_rcu_sched(&freeinit->rcu, do_free_init);
>   	mutex_unlock(&module_mutex);