[PATCH 3.16 294/306] netfilter: nfnetlink: correctly validate length of batch messages

Ben Hutchings
2017-02-16T00:15:55Z

From: Phil Turnbull <phil.turnbull@oracle.com>

commit c58d6c93680f28ac58984af61d0a7ebf4319c241 upstream.

If nlh->nlmsg_len is zero then an infinite loop is triggered because
'skb_pull(skb, msglen);' pulls zero bytes.

The calculation in nlmsg_len() underflows if 'nlh->nlmsg_len <
NLMSG_HDRLEN' which bypasses the length validation and will later
trigger an out-of-bound read.

If the length validation does fail then the malformed batch message is
copied back to userspace. However, we cannot do this because the
nlh->nlmsg_len can be invalid. This leads to an out-of-bounds read in
netlink_ack:

[ 41.455421] ==================================================================
[ 41.456431] BUG: KASAN: slab-out-of-bounds in memcpy+0x1d/0x40 at addr ffff880119e79340
[ 41.456431] Read of size 4294967280 by task a.out/987
[ 41.456431] =============================================================================
[ 41.456431] BUG kmalloc-512 (Not tainted): kasan: bad access detected
[ 41.456431] -----------------------------------------------------------------------------
...
[ 41.456431] Bytes b4 ffff880119e79310: 00 00 00 00 d5 03 00 00 b0 fb fe ff 00 00 00 00 ................
[ 41.456431] Object ffff880119e79320: 20 00 00 00 10 00 05 00 00 00 00 00 00 00 00 00 ...............
[ 41.456431] Object ffff880119e79330: 14 00 0a 00 01 03 fc 40 45 56 11 22 33 10 00 05 .......@EV."3...
[ 41.456431] Object ffff880119e79340: f0 ff ff ff 88 99 aa bb 00 14 00 0a 00 06 fe fb ................
^^ start of batch nlmsg with
nlmsg_len=4294967280
...
[ 41.456431] Memory state around the buggy address:
[ 41.456431] ffff880119e79400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 41.456431] ffff880119e79480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 41.456431] >ffff880119e79500: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
[ 41.456431] ^
[ 41.456431] ffff880119e79580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 41.456431] ffff880119e79600: fc fc fc fc fc fc fc fc fc fc fb fb fb fb fb fb
[ 41.456431] ==================================================================

Fix this with better validation of nlh->nlmsg_len and by setting
NFNL_BATCH_FAILURE if any batch message fails length validation.

CAP_NET_ADMIN is required to trigger the bugs.

Fixes: 9ea2aa8b7dba ("netfilter: nfnetlink: validate nfnetlink header from batch")
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[bwh: Backported to 3.16:
- We don't have an error list so don't call nfnl_err_reset()
- Set 'success' variable instead of 'status']
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -273,10 +273,11 @@ replay:
nlh = nlmsg_hdr(skb);
err = 0;

- if (nlmsg_len(nlh) < sizeof(struct nfgenmsg) ||
- skb->len < nlh->nlmsg_len) {
- err = -EINVAL;
- goto ack;
+ if (nlh->nlmsg_len < NLMSG_HDRLEN ||
+ skb->len < nlh->nlmsg_len ||
+ nlmsg_len(nlh) < sizeof(struct nfgenmsg)) {
+ success = false;
+ goto done;
}

/* Only requests are handled by the kernel */