[PATCH] arm64: mm: Fix false positives in W+X checking

[Will Deacon]

Hi Jeffrey,

Thanks for the patch. Comment below.

On Wed, Apr 25, 2018 at 08:13:31AM -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.
> 
> Address the race by flushing the queued work before running
> debug_checkwx().
> 
> Reported-by: Timur Tabi <timur at codeaurora.org>
> Reported-by: Jan Glauber <jan.glauber at caviumnetworks.com>
> Fixes: 1404d6f13e47 ("arm64: dump: Add checking for writable and exectuable pages")
> Signed-off-by: Jeffrey Hugo <jhugo at codeaurora.org>
> ---
>  arch/arm64/mm/mmu.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 2dbb2c9..40d45fd 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -503,6 +503,12 @@ void mark_rodata_ro(void)
>  	update_mapping_prot(__pa_symbol(__start_rodata), (unsigned long)__start_rodata,
>  			    section_size, PAGE_KERNEL_RO);
>  
> +	/*
> +	 * 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.
> +	 */
> +	rcu_barrier_sched();
>  	debug_checkwx();
>  }

Whilst this looks correct to me, it looks to me like other architectures
(e.g. x86) would be affected by this problem as well. Perhaps it would be
better to solve it in the core code before invoking mark_rodata_ro, or is
there some reason that we only run into this on arm64?

Cheers,

Will