[PATCH] tracing: Fix crash when it fails to alloc ring buffer

[Chunyan Zhang]

From: Jing Xia <jing....@spreadtrum.com>

Double free of the ring buffer happens when it fails to alloc new
ring buffer instance for max_buffer if TRACER_MAX_TRACE is configured.
The root cause is that the pointer is not set to NULL after the buffer
is freed in allocate_trace_buffers(), and the freeing of the ring
buffer is invoked again later if the pointer is not equal to Null,
as:

instance_mkdir()
    |-allocate_trace_buffers()
        |-allocate_trace_buffer(tr, &tr->trace_buffer...)
        |-allocate_trace_buffer(tr, &tr->max_buffer...)

          // allocate fail(-ENOMEM),first free
          // and the buffer pointer is not set to null
        |-ring_buffer_free(tr->trace_buffer.buffer)

       // out_free_tr
    |-free_trace_buffers()
        |-free_trace_buffer(&tr->trace_buffer);

              //if trace_buffer is not null, free again
            |-ring_buffer_free(buf->buffer)
                |-rb_free_cpu_buffer(buffer->buffers[cpu])
                    // ring_buffer_per_cpu is null, and
                    // crash in ring_buffer_per_cpu->pages

Signed-off-by: Jing Xia <jing....@spreadtrum.com>
Signed-off-by: Chunyan Zhang <chunyan.zh...@spreadtrum.com>
---
 kernel/trace/trace.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 73e67b6..ed1c0d1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7622,7 +7622,9 @@ static int allocate_trace_buffers(struct trace_array *tr, 
int size)
                                    allocate_snapshot ? size : 1);
        if (WARN_ON(ret)) {
                ring_buffer_free(tr->trace_buffer.buffer);
+               tr->trace_buffer.buffer = NULL;
                free_percpu(tr->trace_buffer.data);
+               tr->trace_buffer.data = NULL;
                return -ENOMEM;
        }
        tr->allocated_snapshot = allocate_snapshot;
-- 
2.7.4