[PATCH bpf-next v6 2/2] bpf: fix stackmap overflow check in __bpf_get_stackid()

[Arnaud Lecomte <contact@arnaud-lcm.com>]

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 v5: https://lore.kernel.org/all/20250826212229.143230-1-contact@arnaud-lcm.com/

Reported-by: syzbot+c9b724fbb41cf2538b7b@syzkaller.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@arnaud-lcm.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/stackmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 1ebc525b7c2f..8b2dcb8a6dc3 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -251,8 +251,9 @@ static long __bpf_get_stackid(struct bpf_map *map,
 {
 	struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
 	struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
+	u32 hash, id, trace_nr, trace_len, i, max_depth;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
-	u32 hash, id, trace_nr, trace_len, i;
+	u32 elem_size = stack_map_data_size(map);
 	bool user = flags & BPF_F_USER_STACK;
 	u64 *ips;
 	bool hash_matches;
@@ -261,8 +262,12 @@ static long __bpf_get_stackid(struct bpf_map *map,
 		/* skipping more than usable stack trace */
 		return -EFAULT;
 
+	max_depth =
+		stack_map_calculate_max_depth(map->value_size, elem_size, flags);
 	trace_nr = trace->nr - skip;
+	trace_nr = min_t(u32, trace_nr, max_depth - skip);
 	trace_len = trace_nr * sizeof(u64);
+
 	ips = trace->ip + skip;
 	hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
 	id = hash & (smap->n_buckets - 1);
-- 
2.47.3