ring-buffer: Use kaslr address instead of text delta

Instead of saving off the text and data pointers and using them to compare
with the current boot's text and data pointers, just save off the KASLR
offset. Then that can be used to figure out how to read the previous boots
buffer.

The last_boot_info will now show this offset, but only if it is for a
previous boot:

  ~# cat instances/boot_mapped/last_boot_info
  39000000	[kernel]

  ~# echo function > instances/boot_mapped/current_tracer
  ~# cat instances/boot_mapped/last_boot_info
  # Current

If the KASLR offset saved is for the current boot, the last_boot_info will
show the value of "current".

Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

include/linux/ring_buffer.h

@@ -94,8 +94,7 @@ struct trace_buffer *__ring_buffer_alloc_range(unsigned long size, unsigned flag
 					       unsigned long range_size,
 					       struct lock_class_key *key);
 
-bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, long *text,
-				 long *data);
+bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, unsigned long *kaslr_addr);
 
 /*
  * Because the ring buffer is generic, if other users of the ring buffer get

kernel/trace/ring_buffer.c

@@ -31,6 +31,7 @@
 
 #include <asm/local64.h>
 #include <asm/local.h>
+#include <asm/setup.h>
 
 #include "trace.h"
 
@@ -49,8 +50,7 @@ static void update_pages_handler(struct work_struct *work);
 struct ring_buffer_meta {
 	int		magic;
 	int		struct_size;
-	unsigned long	text_addr;
-	unsigned long	data_addr;
+	unsigned long	kaslr_addr;
 	unsigned long	first_buffer;
 	unsigned long	head_buffer;
 	unsigned long	commit_buffer;
@@ -550,8 +550,7 @@ struct trace_buffer {
 	unsigned long			range_addr_start;
 	unsigned long			range_addr_end;
 
-	long				last_text_delta;
-	long				last_data_delta;
+	unsigned long			kaslr_addr;
 
 	unsigned int			subbuf_size;
 	unsigned int			subbuf_order;
@@ -1891,16 +1890,13 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
 	}
 }
 
-/* Used to calculate data delta */
-static char rb_data_ptr[] = "";
-
-#define THIS_TEXT_PTR		((unsigned long)rb_meta_init_text_addr)
-#define THIS_DATA_PTR		((unsigned long)rb_data_ptr)
-
 static void rb_meta_init_text_addr(struct ring_buffer_meta *meta)
 {
-	meta->text_addr = THIS_TEXT_PTR;
-	meta->data_addr = THIS_DATA_PTR;
+#ifdef CONFIG_RANDOMIZE_BASE
+	meta->kaslr_addr = kaslr_offset();
+#else
+	meta->kaslr_addr = 0;
+#endif
 }
 
 static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
@@ -1928,8 +1924,7 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
 			meta->first_buffer += delta;
 			meta->head_buffer += delta;
 			meta->commit_buffer += delta;
-			buffer->last_text_delta = THIS_TEXT_PTR - meta->text_addr;
-			buffer->last_data_delta = THIS_DATA_PTR - meta->data_addr;
+			buffer->kaslr_addr = meta->kaslr_addr;
 			continue;
 		}
 
@@ -2482,17 +2477,15 @@ struct trace_buffer *__ring_buffer_alloc_range(unsigned long size, unsigned flag
  *
  * Returns: The true if the delta is non zero
  */
-bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, long *text,
-				 long *data)
+bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, unsigned long *kaslr_addr)
 {
 	if (!buffer)
 		return false;
 
-	if (!buffer->last_text_delta)
+	if (!buffer->kaslr_addr)
 		return false;
 
-	*text = buffer->last_text_delta;
-	*data = buffer->last_data_delta;
+	*kaslr_addr = buffer->kaslr_addr;
 
 	return true;
 }

kernel/trace/trace.c

@@ -50,7 +50,7 @@
 #include <linux/irq_work.h>
 #include <linux/workqueue.h>
 
-#include <asm/setup.h> /* COMMAND_LINE_SIZE */
+#include <asm/setup.h> /* COMMAND_LINE_SIZE and kaslr_offset() */
 
 #include "trace.h"
 #include "trace_output.h"
@@ -4193,7 +4193,7 @@ static enum print_line_t print_trace_fmt(struct trace_iterator *iter)
 		 * safe to use if the array has delta offsets
 		 * Force printing via the fields.
 		 */
-		if ((tr->text_delta || tr->data_delta) &&
+		if ((tr->text_delta) &&
 		    event->type > __TRACE_LAST_TYPE)
 			return print_event_fields(iter, event);
 
@@ -5990,7 +5990,7 @@ ssize_t tracing_resize_ring_buffer(struct trace_array *tr,
 
 static void update_last_data(struct trace_array *tr)
 {
-	if (!tr->text_delta && !tr->data_delta)
+	if (!(tr->flags & TRACE_ARRAY_FL_LAST_BOOT))
 		return;
 
 	/*
@@ -6003,7 +6003,8 @@ static void update_last_data(struct trace_array *tr)
 
 	/* Using current data now */
 	tr->text_delta = 0;
-	tr->data_delta = 0;
+
+	tr->flags &= ~TRACE_ARRAY_FL_LAST_BOOT;
 }
 
 /**
@@ -6821,8 +6822,17 @@ tracing_last_boot_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t
 
 	seq_buf_init(&seq, buf, 64);
 
-	seq_buf_printf(&seq, "text delta:\t%ld\n", tr->text_delta);
-	seq_buf_printf(&seq, "data delta:\t%ld\n", tr->data_delta);
+	/*
+	 * Do not leak KASLR address. This only shows the KASLR address of
+	 * the last boot. When the ring buffer is started, the LAST_BOOT
+	 * flag gets cleared, and this should only report "current".
+	 * Otherwise it shows the KASLR address from the previous boot which
+	 * should not be the same as the current boot.
+	 */
+	if (tr->flags & TRACE_ARRAY_FL_LAST_BOOT)
+		seq_buf_printf(&seq, "%lx\t[kernel]\n", tr->kaslr_addr);
+	else
+		seq_buf_puts(&seq, "# Current\n");
 
 	return simple_read_from_buffer(ubuf, cnt, ppos, buf, seq_buf_used(&seq));
 }
@@ -9210,8 +9220,10 @@ allocate_trace_buffer(struct trace_array *tr, struct array_buffer *buf, int size
 						      tr->range_addr_start,
 						      tr->range_addr_size);
 
-		ring_buffer_last_boot_delta(buf->buffer,
-					    &tr->text_delta, &tr->data_delta);
+#ifdef CONFIG_RANDOMIZE_BASE
+		if (ring_buffer_last_boot_delta(buf->buffer, &tr->kaslr_addr))
+			tr->text_delta = kaslr_offset() - tr->kaslr_addr;
+#endif
 		/*
 		 * This is basically the same as a mapped buffer,
 		 * with the same restrictions.
@@ -10459,7 +10471,7 @@ __init static void enable_instances(void)
 		 * to it.
 		 */
 		if (start) {
-			tr->flags |= TRACE_ARRAY_FL_BOOT;
+			tr->flags |= TRACE_ARRAY_FL_BOOT | TRACE_ARRAY_FL_LAST_BOOT;
 			tr->ref++;
 		}
 

kernel/trace/trace.h

@@ -348,8 +348,8 @@ struct trace_array {
 	unsigned int		mapped;
 	unsigned long		range_addr_start;
 	unsigned long		range_addr_size;
+	unsigned long		kaslr_addr;
 	long			text_delta;
-	long			data_delta;
 
 	struct trace_pid_list	__rcu *filtered_pids;
 	struct trace_pid_list	__rcu *filtered_no_pids;
@@ -433,9 +433,10 @@ struct trace_array {
 };
 
 enum {
-	TRACE_ARRAY_FL_GLOBAL	= BIT(0),
-	TRACE_ARRAY_FL_BOOT	= BIT(1),
-	TRACE_ARRAY_FL_MOD_INIT	= BIT(2),
+	TRACE_ARRAY_FL_GLOBAL		= BIT(0),
+	TRACE_ARRAY_FL_BOOT		= BIT(1),
+	TRACE_ARRAY_FL_LAST_BOOT	= BIT(2),
+	TRACE_ARRAY_FL_MOD_INIT		= BIT(3),
 };
 
 #ifdef CONFIG_MODULES