[for-linus][PATCH 09/12] ring-buffer: Cap static ring buffer nr_pages

[Steven Rostedt]

From: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

Static ring buffers (i.e. persistent, user-mapped and remote) rely on
the bpage::id field. The number of pages for those ring buffers must fit
into that variable. Enforce this limit on ring buffer creation or
user-mapping.

While at it, prevent nr_pages underflow when allocating a persistent
buffer.

Link: https://patch.msgid.link/20260904164450.1345852-4-vdonnefort@xxxxxxxxxx
Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 077d6940af0c..76fed01f1c49 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -657,6 +657,15 @@ static bool rb_is_static(struct ring_buffer_per_cpu *cpu_buffer)
 	return cpu_buffer->user_mapped || cpu_buffer->remote || cpu_buffer->ring_meta;
 }
 
+static unsigned long rb_static_max_pages(void)
+{
+	/*
+	 * Static ring buffers are using bpage::id and must account for the
+	 * reader page.
+	 */
+	return (1UL << 30) - 1;
+}
+
 struct ring_buffer_iter {
 	struct ring_buffer_per_cpu	*cpu_buffer;
 	unsigned long			head;
@@ -2838,6 +2847,8 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		size = end - buffers_start;
 		size = size / nr_cpu_ids;
 
+		if (size < sizeof(struct ring_buffer_cpu_meta))
+			goto fail_free_buffers;
 		/*
 		 * The number of sub-buffers (nr_pages) is determined by the
 		 * total size allocated minus the meta data size.
@@ -2847,6 +2858,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		 */
 		nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
 			(subbuf_size + sizeof(int));
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+
 		/* Need at least two pages plus the reader page */
 		if (nr_pages < 3)
 			goto fail_free_buffers;
@@ -2879,6 +2894,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		/* The writer is remote. This ring-buffer is read-only */
 		atomic_inc(&buffer->record_disabled);
 		nr_pages = desc->nr_page_va - 1;
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+
 		if (nr_pages < 2)
 			goto fail_free_buffers;
 	} else {
@@ -7841,6 +7860,9 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 	/* prevent another thread from changing buffer/sub-buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
+	if (cpu_buffer->nr_pages > rb_static_max_pages())
+		return -E2BIG;
+
 	err = rb_alloc_meta_page(cpu_buffer);
 	if (err)
 		return err;
-- 
2.53.0