[PATCH 4.9 51/54] macsec: avoid heap overflow in skb_to_sgvec

From: Greg Kroah-Hartman
Date: Mon May 01 2017 - 17:51:11 EST

4.9-stable review patch. If anyone has any objections, please let me know.


From: Jason A. Donenfeld <Jason@xxxxxxxxx>

commit 4d6fa57b4dab0d77f4d8e9d9c73d1e63f6fe8fee upstream.

While this may appear as a humdrum one line change, it's actually quite
important. An sk_buff stores data in three places:

1. A linear chunk of allocated memory in skb->data. This is the easiest
one to work with, but it precludes using scatterdata since the memory
must be linear.
2. The array skb_shinfo(skb)->frags, which is of maximum length
MAX_SKB_FRAGS. This is nice for scattergather, since these fragments
can point to different pages.
3. skb_shinfo(skb)->frag_list, which is a pointer to another sk_buff,
which in turn can have data in either (1) or (2).

The first two are rather easy to deal with, since they're of a fixed
maximum length, while the third one is not, since there can be
potentially limitless chains of fragments. Fortunately dealing with
frag_list is opt-in for drivers, so drivers don't actually have to deal
with this mess. For whatever reason, macsec decided it wanted pain, and
so it explicitly specified NETIF_F_FRAGLIST.

Because dealing with (1), (2), and (3) is insane, most users of sk_buff
doing any sort of crypto or paging operation calls a convenient function
called skb_to_sgvec (which happens to be recursive if (3) is in use!).
This takes a sk_buff as input, and writes into its output pointer an
array of scattergather list items. Sometimes people like to declare a
fixed size scattergather list on the stack; othertimes people like to
allocate a fixed size scattergather list on the heap. However, if you're
doing it in a fixed-size fashion, you really shouldn't be using
NETIF_F_FRAGLIST too (unless you're also ensuring the sk_buff and its
frag_list children arent't shared and then you check the number of
fragments in total required.)

Macsec specifically does this:

size += sizeof(struct scatterlist) * (MAX_SKB_FRAGS + 1);
tmp = kmalloc(size, GFP_ATOMIC);
*sg = (struct scatterlist *)(tmp + sg_offset);
sg_init_table(sg, MAX_SKB_FRAGS + 1);
skb_to_sgvec(skb, sg, 0, skb->len);

Specifying MAX_SKB_FRAGS + 1 is the right answer usually, but not if you're
using NETIF_F_FRAGLIST, in which case the call to skb_to_sgvec will
overflow the heap, and disaster ensues.

Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/net/macsec.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -2709,7 +2709,7 @@ static netdev_tx_t macsec_start_xmit(str

static struct lock_class_key macsec_netdev_addr_lock_key;

static int macsec_dev_init(struct net_device *dev)