[PATCH] perf: Fix vmalloc ring buffer free function

[Jiri Olsa <jolsa@redhat.com>]

If we allocate perf ring buffer with the size of single page,
we will get memory corruption when releasing it. It's caused
by rb_free_work function (the CONFIG_PERF_USE_VMALLOC option
variant).

For single page sized ring buffer the page_order is -1 (because
nr_pages is 0). This needs to be recognized in the rb_free_work
function and set 'nr' to 0 in this case, so only the user page
gets freed.

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 kernel/events/ring_buffer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 23cb34f..21159fb 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -336,7 +336,8 @@ static void rb_free_work(struct work_struct *work)
 	int i, nr;
 
 	rb = container_of(work, struct ring_buffer, work);
-	nr = 1 << page_order(rb);
+	/* -1 if there's only user page */
+	nr = page_order(rb) == -1 ? 0 : 1 << page_order(rb);
 
 	base = rb->user_page;
 	for (i = 0; i < nr + 1; i++)
-- 
1.7.11.7