[PATCH 5.4 30/39] net: skbuff: ensure LSE is pullable before decrementing the MPLS ttl

[Greg Kroah-Hartman]

From: Davide Caratti <dcaratti@xxxxxxxxxx>

[ Upstream commit 13de4ed9e3a9ccbe54d05f7d5c773f69ecaf6c64 ]

skb_mpls_dec_ttl() reads the LSE without ensuring that it is contained in
the skb "linear" area. Fix this calling pskb_may_pull() before reading the
current ttl.

Found by code inspection.

Fixes: 2a2ea50870ba ("net: sched: add mpls manipulation actions to TC")
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Signed-off-by: Davide Caratti <dcaratti@xxxxxxxxxx>
Link: https://lore.kernel.org/r/53659f28be8bc336c113b5254dc637cc76bbae91.1606987074.git.dcaratti@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/core/skbuff.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5618,6 +5618,9 @@ int skb_mpls_dec_ttl(struct sk_buff *skb
 	if (unlikely(!eth_p_mpls(skb->protocol)))
 		return -EINVAL;
 
+	if (!pskb_may_pull(skb, skb_network_offset(skb) + MPLS_HLEN))
+		return -ENOMEM;
+
 	lse = be32_to_cpu(mpls_hdr(skb)->label_stack_entry);
 	ttl = (lse & MPLS_LS_TTL_MASK) >> MPLS_LS_TTL_SHIFT;
 	if (!--ttl)