ip6_gre: fix ip6gre_err() invalid reads

Andrey Konovalov reported out of bound accesses in ip6gre_err()

If GRE flags contains GRE_KEY, the following expression
*(((__be32 *)p) + (grehlen / 4) - 1)

accesses data ~40 bytes after the expected point, since
grehlen includes the size of IPv6 headers.

Let's use a "struct gre_base_hdr *greh" pointer to make this
code more readable.

p[1] becomes greh->protocol.
grhlen is the GRE header length.

Fixes: c12b395a4664 ("gre: Support GRE over IPv6")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 19 deletions

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 558631860d91..630b73be5999 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -367,35 +367,37 @@ static void ip6gre_tunnel_uninit(struct net_device *dev)
 
 
 static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
-                u8 type, u8 code, int offset, __be32 info)
+                       u8 type, u8 code, int offset, __be32 info)
 {
-        const struct ipv6hdr *ipv6h = (const struct ipv6hdr *)skb->data;
-        __be16 *p = (__be16 *)(skb->data + offset);
-        int grehlen = offset + 4;
+        const struct gre_base_hdr *greh;
+        const struct ipv6hdr *ipv6h;
+        int grehlen = sizeof(*greh);
         struct ip6_tnl *t;
+        int key_off = 0;
         __be16 flags;
+        __be32 key;
 
-        flags = p[0];
-        if (flags&(GRE_CSUM|GRE_KEY|GRE_SEQ|GRE_ROUTING|GRE_VERSION)) {
-                if (flags&(GRE_VERSION|GRE_ROUTING))
-                        return;
-                if (flags&GRE_KEY) {
-                        grehlen += 4;
-                        if (flags&GRE_CSUM)
-                                grehlen += 4;
-                }
+        if (!pskb_may_pull(skb, offset + grehlen))
+                return;
+        greh = (const struct gre_base_hdr *)(skb->data + offset);
+        flags = greh->flags;
+        if (flags & (GRE_VERSION | GRE_ROUTING))
+                return;
+        if (flags & GRE_CSUM)
+                grehlen += 4;
+        if (flags & GRE_KEY) {
+                key_off = grehlen + offset;
+                grehlen += 4;
         }
 
-        /* If only 8 bytes returned, keyed message will be dropped here */
-        if (!pskb_may_pull(skb, grehlen))
+        if (!pskb_may_pull(skb, offset + grehlen))
                 return;
         ipv6h = (const struct ipv6hdr *)skb->data;
-        p = (__be16 *)(skb->data + offset);
+        greh = (const struct gre_base_hdr *)(skb->data + offset);
+        key = key_off ? *(__be32 *)(skb->data + key_off) : 0;
 
         t = ip6gre_tunnel_lookup(skb->dev, &ipv6h->daddr, &ipv6h->saddr,
-                                flags & GRE_KEY ?
-                                *(((__be32 *)p) + (grehlen / 4) - 1) : 0,
-                                p[1]);
+                                 key, greh->protocol);
         if (!t)
                 return;