Re: [PATCH] perf: Fix race in perf_mmap_close function

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Sep 11, 2020 at 12:05:10PM +0900, Namhyung Kim wrote:
> Hi Jiri,
> 
> On Thu, Sep 10, 2020 at 11:50 PM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Thu, Sep 10, 2020 at 10:48:02PM +0900, Namhyung Kim wrote:
> >
> > SNIP
> >
> > > >   _do_fork+0x83/0x3a0
> > > >   __do_sys_wait4+0x83/0x90
> > > >   __do_sys_clone+0x85/0xa0
> > > >   do_syscall_64+0x5b/0x1e0
> > > >   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > >
> > > > Using atomic decrease and check instead of separated calls.
> > > > This fixes CVE-2020-14351.
> > > >
> > > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > > ---
> > > >  kernel/events/core.c | 4 +---
> > > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > > >
> > > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > > index 7ed5248f0445..29313cc54d9e 100644
> > > > --- a/kernel/events/core.c
> > > > +++ b/kernel/events/core.c
> > > > @@ -5903,8 +5903,6 @@ static void perf_mmap_close(struct vm_area_struct *vma)
> > > >                 mutex_unlock(&event->mmap_mutex);
> > > >         }
> > > >
> > > > -       atomic_dec(&rb->mmap_count);
> > > > -
> > > >         if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> > > >                 goto out_put;
> > >
> > > But when it takes the goto, rb->mmap_count won't decrement anymore..
> >
> > event->mmap_count is per event, so if we have have race in here,
> > 2 threads can go through with each event->mmap_count reaching zero
> 
> Maybe I'm missing something.
> 
> But as far as I can see, perf_mmap_close() always decremented both
> rb->mmap_count and event->mmap_count.  But with this change,
> it seems not decrement rb->mmap_count when event->mmap_count
> doesn't go to zero, right?

ugh, that's right.. how about change below

jirka


---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7ed5248f0445..8ab2400aef55 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5868,11 +5868,11 @@ static void perf_pmu_output_stop(struct perf_event *event);
 static void perf_mmap_close(struct vm_area_struct *vma)
 {
 	struct perf_event *event = vma->vm_file->private_data;
-
 	struct perf_buffer *rb = ring_buffer_get(event);
 	struct user_struct *mmap_user = rb->mmap_user;
 	int mmap_locked = rb->mmap_locked;
 	unsigned long size = perf_data_size(rb);
+	bool detach_rest = false;
 
 	if (event->pmu->event_unmapped)
 		event->pmu->event_unmapped(event, vma->vm_mm);
@@ -5903,7 +5903,8 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 		mutex_unlock(&event->mmap_mutex);
 	}
 
-	atomic_dec(&rb->mmap_count);
+	if (atomic_dec_and_test(&rb->mmap_count))
+		detach_rest = true;
 
 	if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
 		goto out_put;
@@ -5912,7 +5913,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 	mutex_unlock(&event->mmap_mutex);
 
 	/* If there's still other mmap()s of this buffer, we're done. */
-	if (atomic_read(&rb->mmap_count))
+	if (!detach_rest)
 		goto out_put;
 
 	/*