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

[Jeffrey Hugo]

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>
---

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);
-- 
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