[PATCH net] net/dccp: fix use-after-free in dccp_invalid_packet

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

pskb_may_pull() can reallocate skb->head, we need to reload dh pointer
in dccp_invalid_packet() or risk use after free.

Bug found by Andrey Konovalov using syzkaller.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Andrey Konovalov <andreyk...@google.com>
---
 net/dccp/ipv4.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index b567c8725aea..edbe59d203ef 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -700,6 +700,7 @@ int dccp_invalid_packet(struct sk_buff *skb)
 {
        const struct dccp_hdr *dh;
        unsigned int cscov;
+       u8 dccph_doff;
 
        if (skb->pkt_type != PACKET_HOST)
                return 1;
@@ -721,18 +722,19 @@ int dccp_invalid_packet(struct sk_buff *skb)
        /*
         * If P.Data Offset is too small for packet type, drop packet and return
         */
-       if (dh->dccph_doff < dccp_hdr_len(skb) / sizeof(u32)) {
-               DCCP_WARN("P.Data Offset(%u) too small\n", dh->dccph_doff);
+       dccph_doff = dh->dccph_doff;
+       if (dccph_doff < dccp_hdr_len(skb) / sizeof(u32)) {
+               DCCP_WARN("P.Data Offset(%u) too small\n", dccph_doff);
                return 1;
        }
        /*
         * If P.Data Offset is too too large for packet, drop packet and return
         */
-       if (!pskb_may_pull(skb, dh->dccph_doff * sizeof(u32))) {
-               DCCP_WARN("P.Data Offset(%u) too large\n", dh->dccph_doff);
+       if (!pskb_may_pull(skb, dccph_doff * sizeof(u32))) {
+               DCCP_WARN("P.Data Offset(%u) too large\n", dccph_doff);
                return 1;
        }
-
+       dh = dccp_hdr(skb);
        /*
         * If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
         * has short sequence numbers), drop packet and return