net: skb_reset_mac_len() must check if mac_header was set

JIRA: https://issues.redhat.com/browse/RHEL-96625
Upstream Status: net-next.git commit 1e4033b

commit 1e4033b
Author: Eric Dumazet <edumazet@google.com>
Date:   Tue Nov 5 17:43:57 2024 +0000

    net: skb_reset_mac_len() must check if mac_header was set

    Recent discussions show that skb_reset_mac_len() should be more careful.

    We expect the MAC header being set.

    If not, clear skb->mac_len and fire a warning for CONFIG_DEBUG_NET=y builds.

    If after investigations we find that not having a MAC header was okay,
    we can remove the warning.

    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Link: https://lore.kernel.org/netdev/CANn89iJZGH+yEfJxfPWa3Hm7jxb-aeY2Up4HufmLMnVuQXt38A@mail.gmail.com/T/
    Cc: En-Wei Wu <en-wei.wu@canonical.com>
    Reviewed-by: Joe Damato <jdamato@fastly.com>
    Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
    Link: https://patch.msgid.link/20241105174403.850330-2-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Davide Caratti <dcaratti@redhat.com>

Author: dcaratti

include/linux/skbuff.h

@@ -2916,9 +2916,19 @@ static inline void skb_reset_inner_headers(struct sk_buff *skb)
 	skb->inner_transport_header = skb->transport_header;
 }
 
+static inline int skb_mac_header_was_set(const struct sk_buff *skb)
+{
+	return skb->mac_header != (typeof(skb->mac_header))~0U;
+}
+
 static inline void skb_reset_mac_len(struct sk_buff *skb)
 {
-	skb->mac_len = skb->network_header - skb->mac_header;
+	if (!skb_mac_header_was_set(skb)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		skb->mac_len = 0;
+	} else {
+		skb->mac_len = skb->network_header - skb->mac_header;
+	}
 }
 
 static inline unsigned char *skb_inner_transport_header(const struct sk_buff
@@ -3021,11 +3031,6 @@ static inline void skb_set_network_header(struct sk_buff *skb, const int offset)
 	skb->network_header += offset;
 }
 
-static inline int skb_mac_header_was_set(const struct sk_buff *skb)
-{
-	return skb->mac_header != (typeof(skb->mac_header))~0U;
-}
-
 static inline unsigned char *skb_mac_header(const struct sk_buff *skb)
 {
 	DEBUG_NET_WARN_ON_ONCE(!skb_mac_header_was_set(skb));