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* [PATCH v2] init: Fix false positives in W+X checking
@ 2018-04-27 21:55 Jeffrey Hugo
  2018-04-28  3:05 ` Kees Cook
                   ` (2 more replies)
  0 siblings, 3 replies; 5+ messages in thread
From: Jeffrey Hugo @ 2018-04-27 21:55 UTC (permalink / raw)
  To: linux-arm-kernel, linux-kernel
  Cc: Mark Rutland, Jan Glauber, Kees Cook, Ard Biesheuvel,
	Catalin Marinas, Will Deacon, Laura Abbott, Timur Tabi,
	Stephen Smalley, Andrew Morton, Ingo Molnar, Thomas Gleixner,
	Peter Zijlstra, 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 <>
Reported-by: Jan Glauber <>
Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
Signed-off-by: Jeffrey Hugo <>

-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 +++++++
 1 file changed, 7 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
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Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

^ permalink raw reply	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2018-04-30 14:10 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-04-27 21:55 [PATCH v2] init: Fix false positives in W+X checking Jeffrey Hugo
2018-04-28  3:05 ` Kees Cook
2018-04-28  6:14 ` Ingo Molnar
2018-04-30 14:10   ` Jeffrey Hugo
2018-04-30  8:31 ` Will Deacon

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