net: skbuff: ensure LSE is pullable before decrementing the MPLS ttl

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@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/53659f28be8bc336c113b5254dc637cc76bbae91.1606987074.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 06c526e0d810e4cd948c4d7954d8b5283345e370..e578544b2cc7110ec2f6bcf4c29d93e4b4b1ad14 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5786,6 +5786,9 @@ 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)