2 files changed, 19 insertions, 5 deletions

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index d56a64c99a8b..eb675c4d59df 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -16,7 +16,7 @@ struct ring_buffer {
         int                             page_order;     /* allocation order  */
 #endif
         int                             nr_pages;       /* nr of data pages  */
-        int                             writable;       /* are we writable   */
+        int                             overwrite;      /* can overwrite itself */
 
         atomic_t                        poll;           /* POLL_ for wakeups */
 
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 23cb34ff3973..97fddb09762b 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -18,12 +18,24 @@
 static bool perf_output_space(struct ring_buffer *rb, unsigned long tail,
                               unsigned long offset, unsigned long head)
 {
-        unsigned long mask;
+        unsigned long sz = perf_data_size(rb);
+        unsigned long mask = sz - 1;
 
-        if (!rb->writable)
+        /*
+         * check if user-writable
+         * overwrite : over-write its own tail
+         * !overwrite: buffer possibly drops events.
+         */
+        if (rb->overwrite)
                 return true;
 
-        mask = perf_data_size(rb) - 1;
+        /*
+         * verify that payload is not bigger than buffer
+         * otherwise masking logic may fail to detect
+         * the "not enough space" condition
+         */
+        if ((head - offset) > sz)
+                return false;
 
         offset = (offset - tail) & mask;
         head   = (head   - tail) & mask;
@@ -212,7 +224,9 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
                 rb->watermark = max_size / 2;
 
         if (flags & RING_BUFFER_WRITABLE)
-                rb->writable = 1;
+                rb->overwrite = 0;
+        else
+                rb->overwrite = 1;
 
         atomic_set(&rb->refcount, 1);