linux-kernel - [PATCH] tracing: Fix overflow in get_free

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Message-ID: <20240710091908.7030-1-Tze-nan.Wu@mediatek.com>
Date: Wed, 10 Jul 2024 17:19:05 +0800
From: Tze-nan Wu <Tze-nan.Wu@...iatek.com>
To: <rostedt@...dmis.org>
CC: <cheng-jui.wang@...iatek.com>, <bobule.chang@...iatek.com>,
	<eric-yk.wu@...iatek.com>, <wsd_upstream@...iatek.com>, Tze-nan Wu
	<Tze-nan.Wu@...iatek.com>, Mathieu Desnoyers
	<mathieu.desnoyers@...icios.com>, <linux-kernel@...r.kernel.org>,
	<linux-trace-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-mediatek@...ts.infradead.org>
Subject: [PATCH] tracing: Fix overflow in get_free_elt()

"tracing_map->next_elt" in get_free_elt() is at risk of overflowing.

Once it overflows, new elements can still be inserted into the tracing_map
even though the maximum number of elements (`max_elts`) has been reached.
Continuing to insert elements after the overflow could result in the
tracing_map containing "tracing_map->max_size" elements, leaving no empty
entries.
If any attempt is made to insert an element into a full tracing_map using
`__tracing_map_insert()`, it will cause an infinite loop with preemption
disabled, leading to a CPU hang problem.

Fix this by preventing any further increments to "tracing_map->next_elt"
once it reaches "tracing_map->max_elt".

Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@...iatek.com>
---
 kernel/trace/tracing_map.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/tracing_map.c b/kernel/trace/tracing_map.c
index a4dcf0f24352..3a56e7c8aa4f 100644
--- a/kernel/trace/tracing_map.c
+++ b/kernel/trace/tracing_map.c
@@ -454,7 +454,7 @@ static struct tracing_map_elt *get_free_elt(struct tracing_map *map)
 	struct tracing_map_elt *elt = NULL;
 	int idx;
 
-	idx = atomic_inc_return(&map->next_elt);
+	idx = atomic_fetch_add_unless(&map->next_elt, 1, map->max_elts);
 	if (idx < map->max_elts) {
 		elt = *(TRACING_MAP_ELT(map->elts, idx));
 		if (map->ops && map->ops->elt_init)
@@ -699,7 +699,7 @@ void tracing_map_clear(struct tracing_map *map)
 {
 	unsigned int i;
 
-	atomic_set(&map->next_elt, -1);
+	atomic_set(&map->next_elt, 0);
 	atomic64_set(&map->hits, 0);
 	atomic64_set(&map->drops, 0);
 
@@ -783,7 +783,7 @@ struct tracing_map *tracing_map_create(unsigned int map_bits,
 
 	map->map_bits = map_bits;
 	map->max_elts = (1 << map_bits);
-	atomic_set(&map->next_elt, -1);
+	atomic_set(&map->next_elt, 0);
 
 	map->map_size = (1 << (map_bits + 1));
 	map->ops = ops;
-- 
2.18.0