Linux-ARM-Kernel Archive on
 help / color / Atom feed
From: (Jeffrey Hugo)
Subject: [PATCH v3] init: Fix false positives in W+X checking
Date: Mon, 30 Apr 2018 09:59:06 -0600
Message-ID: <> (raw)

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 <>
Reported-by: Jan Glauber <>
Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
Signed-off-by: Jeffrey Hugo <>
Acked-by: Kees Cook <>
Acked-by: Ingo Molnar <>
Acked-by: Will Deacon <>

-added comment to module code to establish matching connection
-added acks by Kees, Ingo, and Will

-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


 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();
 	} 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);
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

             reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-30 15:59 jhugo [this message]
2018-04-30 17:19 ` labbott
2018-04-30 18:40   ` keescook
2018-04-30 20:49     ` jhugo

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-ARM-Kernel Archive on

Archives are clonable:
	git clone --mirror linux-arm-kernel/git/0.git
	git clone --mirror linux-arm-kernel/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-arm-kernel linux-arm-kernel/ \
	public-inbox-index linux-arm-kernel

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone