ACK: [PATCH] init: fix false positives in W+X checking

[Kleber Souza]

On 05/15/18 16:36, Manoj Iyer wrote:
> From: Jeffrey Hugo <jhugo at codeaurora.org>
> 
> 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().
> 
> Link: http://lkml.kernel.org/r/1525103946-29526-1-git-send-email-jhugo@codeaurora.org
> Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> 
> BugLink: https://launchpad.net/bugs/1769696
> 
> Signed-off-by: Jeffrey Hugo <jhugo at codeaurora.org>
> Reported-by: Timur Tabi <timur at codeaurora.org>
> Reported-by: Jan Glauber <jan.glauber at caviumnetworks.com>
> Acked-by: Kees Cook <keescook at chromium.org>
> Acked-by: Ingo Molnar <mingo at kernel.org>
> Acked-by: Will Deacon <will.deacon at arm.com>
> Acked-by: Laura Abbott <labbott at redhat.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Stephen Smalley <sds at tycho.nsa.gov>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (cherry picked from commit ae646f0b9ca135b87bc73ff606ef996c3029780a)
> Signed-off-by: Manoj Iyer <manoj.iyer at canonical.com>

Clean cherry-pick from upstream:

Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>

> ---
>  init/main.c     | 7 +++++++
>  kernel/module.c | 5 +++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/init/main.c b/init/main.c
> index b8b121c17ff1..44f88af9b191 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -980,6 +980,13 @@ __setup("rodata=", set_debug_rodata);
>  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 2612f760df84..0da7f3468350 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3517,6 +3517,11 @@ 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);
>