[PATCH] ring-buffer: Check resize_disabled before publishing the new subbuf order

[David Carlier <devnexen@gmail.com>]

ring_buffer_subbuf_order_set() stores the new order and only then walks
the CPUs, returning -EBUSY if any of them has resizing disabled. A user
mapped buffer has resizing disabled, and __rb_map_vma() reads
buffer->subbuf_order without buffer->mutex, so an mmap of an already
mapped CPU racing the failing order change sizes the mapping with the
new order and inserts pages past the sub-buffer into the VMA.

Check the CPUs before storing the new order.

Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
Cc: stable@vger.kernel.org
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 kernel/trace/ring_buffer.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c03a555a6ba..d7e5e4620d09 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7474,6 +7474,14 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 
 	old_capacity = rb_subbuf_capacity(buffer);
 
+	/* The mmap fast path reads subbuf_order without buffer->mutex. */
+	for_each_buffer_cpu(buffer, cpu) {
+		if (!cpumask_test_cpu(cpu, buffer->cpumask))
+			continue;
+		if (atomic_read(&buffer->buffers[cpu]->resize_disabled))
+			return -EBUSY;
+	}
+
 	atomic_inc(&buffer->record_disabled);
 
 	/* Make sure all commits have finished */
-- 
2.55.0