[RFC PATCH] perf/core: Prevent dereferencing NULL pointer

[I Hsin Cheng <richard120310@gmail.com>]

According to coverity scan check, there's possible cases where
"ring_buffer_get()" returns a NULL in "perf_mmap_close".

Use a "BUG_ON()" to check for NULL pointer existence, panic if it does
exist, otherwise it's safe to dereference "rb" and access its members.

The scan check report link is:
scan5.scan.coverity.com/#/project-view/63416/10063?selectedIssue=1636067

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
---
 kernel/events/core.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index bcb09e011e9e..fe83d4754746 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6410,11 +6410,17 @@ 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);
+	struct user_struct *mmap_user;
+	int mmap_locked;
+	unsigned long size;
 	bool detach_rest = false;
 
+	BUG_ON(!rb);
+
+	mmap_user = rb->mmap_user;
+	mmap_locked = rb->mmap_locked;
+	size = perf_data_size(rb);
+
 	if (event->pmu->event_unmapped)
 		event->pmu->event_unmapped(event, vma->vm_mm);
 
-- 
2.43.0