[for-linus][PATCH 08/20] tracing: Keep the entry count when the histogram stats allocation fails

[Steven Rostedt <rostedt@kernel.org>]

From: Donggeun Yoo <donggeunyoo.kernel@gmail.com>

print_entries() uses n_entries both as the number of sort entries and as
its own return value, so the -ENOMEM it stores when the stats allocation
fails overwrites the count that the cleanup still needs:

	n_entries = tracing_map_sort_entries(map, ...);
	if (n_entries < 0)
		return n_entries;
	...
			if (!stats) {
				n_entries = -ENOMEM;
				goto out;
			}
	...
 out:
	tracing_map_destroy_sort_entries(sort_entries, n_entries);

tracing_map_destroy_sort_entries() takes an unsigned int and loops up to
it, so -ENOMEM arrives as 4294967284. It walks an array of at most
map->max_elts pointers and calls destroy_sort_entry(), which dereferences
and frees, on whatever lies past the end.

Reading the hist file of a trigger with a .percent value, with that
allocation forced to fail:

  BUG: KASAN: vmalloc-out-of-bounds in tracing_map_destroy_sort_entries+0xa0/0xb0
  Read of size 8 at addr ffffc90000045000 by task init/1
   tracing_map_destroy_sort_entries+0xa0/0xb0
   hist_show+0x6f7/0x1df0
   seq_read_iter+0x2b8/0x1190
   vfs_read+0x176/0xa40
  The buggy address belongs to a 4-page vmalloc region starting at
  ffffc90000041000 allocated at tracing_map_sort_entries+0x5c/0xd50

A few pages further the fault is fatal. The registers at the oops confirm
the bound: the loop's end pointer less the array start, over the pointer
size, is 4294967284.

Return the error in a separate variable and leave n_entries holding the
count, the way tracing_map_sort_entries() does on its own error path.

The stats block is only entered for a value carrying .percent or .graph,
which __create_val_field() has rejected since v6.3, so this cannot be
reached in mainline as it stands. It becomes reachable again with
"tracing: hist: let values keep the percent and graph modifiers", so it
should be applied first.

Cc: stable@vger.kernel.org
Fixes: abaa5258ce5e ("tracing: Add .percent suffix option to histogram values")
Link: https://patch.msgid.link/20260907060323.480728-1-donggeunyoo.kernel@gmail.com
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260907053113.1CED91F00A3A@smtp.kernel.org/
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/trace_events_hist.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 8cad99a8d01e..8d80562fb502 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -5677,7 +5677,7 @@ static int print_entries(struct seq_file *m,
 {
 	struct tracing_map_sort_entry **sort_entries = NULL;
 	struct tracing_map *map = hist_data->map;
-	int i, j, n_entries;
+	int i, j, n_entries, ret;
 	struct hist_val_stat *stats = NULL;
 	u64 val;
 
@@ -5687,6 +5687,8 @@ static int print_entries(struct seq_file *m,
 	if (n_entries < 0)
 		return n_entries;
 
+	ret = n_entries;
+
 	/* Calculate the max and the total for each field if needed. */
 	for (j = 0; j < hist_data->n_vals; j++) {
 		if (!(hist_data->fields[j]->flags &
@@ -5695,7 +5697,7 @@ static int print_entries(struct seq_file *m,
 		if (!stats) {
 			stats = kzalloc_objs(*stats, hist_data->n_vals);
 			if (!stats) {
-				n_entries = -ENOMEM;
+				ret = -ENOMEM;
 				goto out;
 			}
 		}
@@ -5716,7 +5718,7 @@ static int print_entries(struct seq_file *m,
 out:
 	tracing_map_destroy_sort_entries(sort_entries, n_entries);
 
-	return n_entries;
+	return ret;
 }
 
 static void hist_trigger_show(struct seq_file *m,
-- 
2.53.0