A DSA frame with an invalid source trunk ID could cause an out-of-bounds
read access of dst->lags.

Add a check to dsa_lag_by_id() to validate the LAG ID is not zero, and is
less than or equal to dst->lags_len. (The LAG ID is derived by adding one
to the source trunk ID.)

Note: this is in the fast path for any frames within a trunk.

Fixes: 5b60dadb71db ("net: dsa: tag_dsa: Support reception of packets from LAG devices")
Signed-off-by: Luke Howard <redacted>
---
 include/net/dsa.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/net/dsa.h b/include/net/dsa.h
index af16347a3d331..9f01ea5bbd877 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h

@@ -190,6 +190,9 @@ struct dsa_switch_tree {
 static inline struct dsa_lag *dsa_lag_by_id(struct dsa_switch_tree *dst,
 					    unsigned int id)
 {
+	if (unlikely(id == 0 || id > dst->lags_len))
+		return NULL;
+
 	/* DSA LAG IDs are one-based, dst->lags is zero-based */
 	return dst->lags[id - 1];
 }

-- 
2.43.0