diff mbox series

[06/17] kthread: cfi: disable callback pointer check with modules

Message ID 20210312004919.669614-7-samitolvanen@google.com (mailing list archive)
State New
Headers show
Series Add support for Clang CFI | expand

Commit Message

Sami Tolvanen March 12, 2021, 12:49 a.m. UTC
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@google.com>
---
 kernel/kthread.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Comments

Kees Cook March 12, 2021, 2:43 a.m. UTC | #1
On Thu, Mar 11, 2021 at 04:49:08PM -0800, Sami Tolvanen wrote:
> 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@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>
Christoph Hellwig March 12, 2021, 6:13 a.m. UTC | #2
On Thu, Mar 11, 2021 at 04:49:08PM -0800, Sami Tolvanen wrote:
> 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@google.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,

you keep spewing this comment line that has exactly 81 characters and
thus badly messes up read it with a normal termina everywhere.

Maybe instead of fixing that in ever duplication hide the whole check in
a well documented helper (which would have to be a macro due to the
typing involved).
Sami Tolvanen March 17, 2021, 4:05 p.m. UTC | #3
On Thu, Mar 11, 2021 at 10:13 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Thu, Mar 11, 2021 at 04:49:08PM -0800, Sami Tolvanen wrote:
> > 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@google.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,
>
> you keep spewing this comment line that has exactly 81 characters and
> thus badly messes up read it with a normal termina everywhere.
>
> Maybe instead of fixing that in ever duplication hide the whole check in
> a well documented helper (which would have to be a macro due to the
> typing involved).

Sure, that sounds cleaner. I'll add a helper macro in v2.

Sami
diff mbox series

Patch

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