netdev - Re: [PATCH v2 net-next] net: dsa: validate source trunk against lags

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Message-Id: <5E37B372-015B-4B19-92E9-7212C33D59C5@padl.com>
Date: Thu, 31 Jul 2025 19:56:37 +1000
From: Luke Howard <lukeh@...l.com>
To: netdev@...r.kernel.org
Cc: Vladimir Oltean <olteanv@...il.com>, Andrew Lunn <andrew@...n.ch>,
        Ryan Wilkins <Ryan.Wilkins@...osalliance.com>
Subject: Re: [PATCH v2 net-next] net: dsa: validate source trunk against
 lags_len

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 <lukeh@...l.com>
---
 include/net/dsa.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/net/dsa.h b/include/net/dsa.h
index 2e148823c366c..67672c5ff22e5 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -180,6 +180,9 @@ 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