linux-kernel - [PATCH bpf-next v7 3/3] bpf: fix stackmap overflow check in __bpf_get

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Message-Id: <20250903234325.30212-1-contact@arnaud-lcm.com>
Date: Thu,  4 Sep 2025 01:43:25 +0200
From: Arnaud Lecomte <contact@...aud-lcm.com>
To: alexei.starovoitov@...il.com,
	yonghong.song@...ux.dev,
	song@...nel.org
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	daniel@...earbox.net,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	sdf@...ichev.me,
	syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	Arnaud Lecomte <contact@...aud-lcm.com>
Subject: [PATCH bpf-next v7 3/3] bpf: fix stackmap overflow check in
 __bpf_get_stackid()

Syzkaller reported a KASAN slab-out-of-bounds write in __bpf_get_stackid()
when copying stack trace data. The issue occurs when the perf trace
 contains more stack entries than the stack map bucket can hold,
 leading to an out-of-bounds write in the bucket's data array.

Changes in v2:
 - Fixed max_depth names across get stack id

Changes in v4:
 - Removed unnecessary empty line in __bpf_get_stackid

Changs in v6:
 - Added back trace_len computation in __bpf_get_stackid

Link to v6: https://lore.kernel.org/all/20250903135348.97884-1-contact@arnaud-lcm.com/

Reported-by: syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
Fixes: ee2a098851bf ("bpf: Adjust BPF stack helper functions to accommodate skip > 0")
Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
Acked-by: Yonghong Song <yonghong.song@...ux.dev>
---
 kernel/bpf/stackmap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 9f3ae426ddc3..29e05c9ff1bd 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -369,6 +369,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
 {
 	struct perf_event *event = ctx->event;
 	struct perf_callchain_entry *trace;
+	u32 elem_size, max_depth;
 	bool kernel, user;
 	__u64 nr_kernel;
 	int ret;
@@ -390,11 +391,15 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
 		return -EFAULT;
 
 	nr_kernel = count_kernel_ip(trace);
+	elem_size = stack_map_data_size(map);
 
 	if (kernel) {
 		__u64 nr = trace->nr;
 
 		trace->nr = nr_kernel;
+		max_depth =
+			stack_map_calculate_max_depth(map->value_size, elem_size, flags);
+		trace->nr = min_t(u32, nr_kernel, max_depth);
 		ret = __bpf_get_stackid(map, trace, flags);
 
 		/* restore nr */
@@ -407,6 +412,9 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
 			return -EFAULT;
 
 		flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
+		max_depth =
+			stack_map_calculate_max_depth(map->value_size, elem_size, flags);
+		trace->nr = min_t(u32, trace->nr, max_depth);
 		ret = __bpf_get_stackid(map, trace, flags);
 	}
 	return ret;
-- 
2.47.3