linux-hardening - [PATCH 06/17] kthread: cfi: disable callback pointer check with modules

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Message-Id: <20210312004919.669614-7-samitolvanen@google.com>
Date:   Thu, 11 Mar 2021 16:49:08 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        bpf@...r.kernel.org, linux-hardening@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kbuild@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 06/17] kthread: cfi: disable callback pointer check with modules

With CONFIG_CFI_CLANG, a callback function passed to
__kthread_queue_delayed_work from a module points to a jump table
entry defined in the module instead of the one used in the core
kernel, which breaks function address equality in this check:

  WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);

Disable the warning when CFI and modules are enabled.

Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 kernel/kthread.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973c5740..af5fee350586 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -963,7 +963,13 @@ static void __kthread_queue_delayed_work(struct kthread_worker *worker,
 	struct timer_list *timer = &dwork->timer;
 	struct kthread_work *work = &dwork->work;
 
-	WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
+	/*
+	 * With CFI, timer->function can point to a jump table entry in a module,
+	 * which fails the comparison. Disable the warning if CFI and modules are
+	 * both enabled.
+	 */
+	if (!IS_ENABLED(CONFIG_CFI_CLANG) || !IS_ENABLED(CONFIG_MODULES))
+		WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
 
 	/*
 	 * If @delay is 0, queue @dwork->work immediately.  This is for
-- 
2.31.0.rc2.261.g7f71774620-goog