[PATCH 4.2.y-ckt 98/98] mld, igmp: Fix reserved tailroom calculation

Kamal Mostafa
2016-03-15T23:31:32Z

From: Benjamin Poirier <bpoirier@suse.com>

commit 1837b2e2bcd23137766555a63867e649c0b637f0 upstream.

The current reserved_tailroom calculation fails to take hlen and tlen into
account.

skb:
[__hlen__|__data____________|__tlen___|__extra__]
^ ^
head skb_end_offset

In this representation, hlen + data + tlen is the size passed to alloc_skb.
"extra" is the extra space made available in __alloc_skb because of
rounding up by kmalloc. We can reorder the representation like so:

[__hlen__|__data____________|__extra__|__tlen___]
^ ^
head skb_end_offset

The maximum space available for ip headers and payload without
fragmentation is min(mtu, data + extra). Therefore,
reserved_tailroom
= data + extra + tlen - min(mtu, data + extra)
= skb_end_offset - hlen - min(mtu, skb_end_offset - hlen - tlen)
= skb_tailroom - min(mtu, skb_tailroom - tlen) ; after skb_reserve(hlen)

Compare the second line to the current expression:
reserved_tailroom = skb_end_offset - min(mtu, skb_end_offset)
and we can see that hlen and tlen are not taken into account.

The min() in the third line can be expanded into:
if mtu < skb_tailroom - tlen:
reserved_tailroom = skb_tailroom - mtu
else:
reserved_tailroom = tlen

Depending on hlen, tlen, mtu and the number of multicast address records,
the current code may output skbs that have less tailroom than
dev->needed_tailroom or it may output more skbs than needed because not all
space available is used.

Fixes: 4c672e4b ("ipv6: mld: fix add_grhead skb_over_panic for devs with large MTUs")
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
include/linux/skbuff.h | 24 ++++++++++++++++++++++++
net/ipv4/igmp.c | 3 +--
net/ipv6/mcast.c | 3 +--
3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 777b208..365a771 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1793,6 +1793,30 @@ static inline void skb_reserve(struct sk_buff *skb, int len)
skb->tail += len;
}

+/**
+ * skb_tailroom_reserve - adjust reserved_tailroom
+ * @skb: buffer to alter
+ * @mtu: maximum amount of headlen permitted
+ * @needed_tailroom: minimum amount of reserved_tailroom
+ *
+ * Set reserved_tailroom so that headlen can be as large as possible but
+ * not larger than mtu and tailroom cannot be smaller than
+ * needed_tailroom.
+ * The required headroom should already have been reserved before using
+ * this function.
+ */
+static inline void skb_tailroom_reserve(struct sk_buff *skb, unsigned int mtu,
+ unsigned int needed_tailroom)
+{
+ SKB_LINEAR_ASSERT(skb);
+ if (mtu < skb_tailroom(skb) - needed_tailroom)
+ /* use at most mtu */
+ skb->reserved_tailroom = skb_tailroom(skb) - mtu;
+ else
+ /* use up to all available space */
+ skb->reserved_tailroom = needed_tailroom;
+}
+
#define ENCAP_TYPE_ETHER 0
#define ENCAP_TYPE_IPPROTO 1

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 2a2b6a4..a557a01 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -353,9 +353,8 @@ static struct sk_buff *igmpv3_newpack(struct net_device *dev, unsigned int mtu)
skb_dst_set(skb, &rt->dst);
skb->dev = dev;

- skb->reserved_tailroom = skb_end_offset(skb) -
- min(mtu, skb_end_offset(skb));
skb_reserve(skb, hlen);
+ skb_tailroom_reserve(skb, mtu, tlen);

skb_reset_network_header(skb);
pip = ip_hdr(skb);
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 41e3b5e..9a63110 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -1574,9 +1574,8 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
return NULL;

skb->priority = TC_PRIO_CONTROL;
- skb->reserved_tailroom = skb_end_offset(skb) -
- min(mtu, skb_end_offset(skb));
skb_reserve(skb, hlen);
+ skb_tailroom_reserve(skb, mtu, tlen);

if (__ipv6_get_lladdr(idev, &addr_buf, IFA_F_TENTATIVE)) {
/* <draft-ietf-magma-mld-source-05.txt>:
--
2.7.0