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Subject[PATCH] macsec: avoid heap overflow in skb_to_sgvec
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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@zx2c4.com>
Cc: stable@vger.kernel.org
Cc: security@kernel.org
---
drivers/net/macsec.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index ff0a5ed3ca80..dbab05afcdbe 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -2716,7 +2716,7 @@ static netdev_tx_t macsec_start_xmit(struct sk_buff *skb,
}

#define MACSEC_FEATURES \
- (NETIF_F_SG | NETIF_F_HIGHDMA | NETIF_F_FRAGLIST)
+ (NETIF_F_SG | NETIF_F_HIGHDMA)
static struct lock_class_key macsec_netdev_addr_lock_key;

static int macsec_dev_init(struct net_device *dev)
--
2.12.2
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