init: fix false positives in W+X checking

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@codeaurora.org>
Reported-by: Timur Tabi <timur@codeaurora.org>
Reported-by: Jan Glauber <jan.glauber@caviumnetworks.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit ae646f0b9ca135b87bc73ff606ef996c3029780a)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Acked-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/init/main.c b/init/main.c
index bb0896c24c088ce3f035d4d76e93aaa693df5e51..077490cae7b4c53e002511ae50d21891b738d975 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -966,6 +966,13 @@ __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 e5b878b2690678f70acc56fd3c60cddd66dbefba..41b97a191a72ee495de444f04f69d3582ef2a5f7 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3495,6 +3495,11 @@ 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);