init: fix false positives in W+X checking

[ Upstream commit ae646f0b9ca135b87bc73ff606ef996c3029780a ]

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")
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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/init/main.c b/init/main.c
index 2d355a61dfc51a5582f834f4fcca382469c83fb0..0d88f37febcb29d56a1f18297a9b9a63dddb5243 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -974,6 +974,13 @@ __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 690c0651c40f4c2121c0e4227660948ff981bac0..321b0b1f87e7a9359e5b7d37023f693e22c9caed 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3506,6 +3506,11 @@ 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);