[PATCH net v2] netfilter: nat: cope with negative port range

[PATCH net v2] netfilter: nat: cope with negative port range

[Paolo Abeni]

syzbot reported a division by 0 bug in the netfilter nat code:

divide error: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
    (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 4168 Comm: syzkaller034710 Not tainted 4.16.0-rc1+ #309
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:nf_nat_l4proto_unique_tuple+0x291/0x530
net/netfilter/nf_nat_proto_common.c:88
RSP: 0018:ffff8801b2466778 EFLAGS: 00010246
RAX: 000000000000f153 RBX: ffff8801b2466dd8 RCX: ffff8801b2466c7c
RDX: 0000000000000000 RSI: ffff8801b2466c58 RDI: ffff8801db5293ac
RBP: ffff8801b24667d8 R08: ffff8801b8ba6dc0 R09: ffffffff88af5900
R10: ffff8801b24666f0 R11: 0000000000000000 R12: 000000002990f153
R13: 0000000000000001 R14: 0000000000000000 R15: ffff8801b2466c7c
FS:  00000000017e3880(0000) GS:ffff8801db500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000208fdfe4 CR3: 00000001b5340002 CR4: 00000000001606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  dccp_unique_tuple+0x40/0x50 net/netfilter/nf_nat_proto_dccp.c:30
  get_unique_tuple+0xc28/0x1c10 net/netfilter/nf_nat_core.c:362
  nf_nat_setup_info+0x1c2/0xe00 net/netfilter/nf_nat_core.c:406
  nf_nat_redirect_ipv6+0x306/0x730 net/netfilter/nf_nat_redirect.c:124
  redirect_tg6+0x7f/0xb0 net/netfilter/xt_REDIRECT.c:34
  ip6t_do_table+0xc2a/0x1a30 net/ipv6/netfilter/ip6_tables.c:365
  ip6table_nat_do_chain+0x65/0x80 net/ipv6/netfilter/ip6table_nat.c:41
  nf_nat_ipv6_fn+0x594/0xa80 net/ipv6/netfilter/nf_nat_l3proto_ipv6.c:302
  nf_nat_ipv6_local_fn+0x33/0x5d0
net/ipv6/netfilter/nf_nat_l3proto_ipv6.c:407
  ip6table_nat_local_fn+0x2c/0x40 net/ipv6/netfilter/ip6table_nat.c:69
  nf_hook_entry_hookfn include/linux/netfilter.h:120 [inline]
  nf_hook_slow+0xba/0x1a0 net/netfilter/core.c:483
  nf_hook include/linux/netfilter.h:243 [inline]
  NF_HOOK include/linux/netfilter.h:286 [inline]
  ip6_xmit+0x10ec/0x2260 net/ipv6/ip6_output.c:277
  inet6_csk_xmit+0x2fc/0x580 net/ipv6/inet6_connection_sock.c:139
  dccp_transmit_skb+0x9ac/0x10f0 net/dccp/output.c:142
  dccp_connect+0x369/0x670 net/dccp/output.c:564
  dccp_v6_connect+0xe17/0x1bf0 net/dccp/ipv6.c:946
  __inet_stream_connect+0x2d4/0xf00 net/ipv4/af_inet.c:620
  inet_stream_connect+0x58/0xa0 net/ipv4/af_inet.c:684
  SYSC_connect+0x213/0x4a0 net/socket.c:1639
  SyS_connect+0x24/0x30 net/socket.c:1620
  do_syscall_64+0x282/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x26/0x9b
RIP: 0033:0x441c69
RSP: 002b:00007ffe50cc0be8 EFLAGS: 00000217 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: ffffffffffffffff RCX: 0000000000441c69
RDX: 000000000000001c RSI: 00000000208fdfe4 RDI: 0000000000000003
RBP: 00000000006cc018 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000538 R11: 0000000000000217 R12: 0000000000403590
R13: 0000000000403620 R14: 0000000000000000 R15: 0000000000000000
Code: 48 89 f0 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 46 02 00 00 48 8b
45 c8 44 0f b7 20 e8 88 97 04 fd 31 d2 41 0f b7 c4 4c 89 f9 <41> f7 f6 48
c1 e9 03 48 b8 00 00 00 00 00 fc ff df 0f b6 0c 01
RIP: nf_nat_l4proto_unique_tuple+0x291/0x530
net/netfilter/nf_nat_proto_common.c:88 RSP: ffff8801b2466778

The problem is that currently we don't have any check on the
configured port range. A port range == -1 triggers the bug, while
other negative values may require a very long time to complete the
following loop.

Adding the relevant check at parse time could break existing
setup, moreover we would need to read/write such values atomically
to avoid possible transient negative ranges at update time.

This commit addresses the issue swapping the two ends on negative
ranges in nf_nat_l4proto_unique_tuple().

v1 -> v2: use the correct 'Fixes' tag

Fixes: 5b1158e909ec ("[NETFILTER]: Add NAT support for nf_conntrack")
Reported-and-tested-by: syzbot+8012e198bd037f4871e5@syzkaller.appspotmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/netfilter/nf_nat_proto_common.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/netfilter/nf_nat_proto_common.c b/net/netfilter/nf_nat_proto_common.c
index fbce552a796e..a05cce545e98 100644
--- a/net/netfilter/nf_nat_proto_common.c
+++ b/net/netfilter/nf_nat_proto_common.c
@@ -41,7 +41,7 @@ void nf_nat_l4proto_unique_tuple(const struct nf_nat_l3proto *l3proto,
 				 const struct nf_conn *ct,
 				 u16 *rover)
 {
-	unsigned int range_size, min, i;
+	unsigned int range_size, min, max, i;
 	__be16 *portptr;
 	u_int16_t off;
 
@@ -70,8 +70,11 @@ void nf_nat_l4proto_unique_tuple(const struct nf_nat_l3proto *l3proto,
 			range_size = 65535 - 1024 + 1;
 		}
 	} else {
-		min = ntohs(range->min_proto.all);
-		range_size = ntohs(range->max_proto.all) - min + 1;
+		min = ntohs(READ_ONCE(range->min_proto.all));
+		max = ntohs(READ_ONCE(range->max_proto.all));
+		if (unlikely(max < min))
+			swap(min, max);
+		range_size = max - min + 1;
 	}
 
 	if (range->flags & NF_NAT_RANGE_PROTO_RANDOM) {
-- 
2.14.3

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Eric Dumazet]

On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> syzbot reported a division by 0 bug in the netfilter nat code:

...

> Adding the relevant check at parse time could break existing
> setup, moreover we would need to read/write such values atomically
> to avoid possible transient negative ranges at update time.

I do not quite follow why it is so hard to add a check at parse time.

Breaking buggy setups would not be a concern I think.

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Florian Westphal]

Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > syzbot reported a division by 0 bug in the netfilter nat code:
> 
> > Adding the relevant check at parse time could break existing
> > setup, moreover we would need to read/write such values atomically
> > to avoid possible transient negative ranges at update time.
> 
> I do not quite follow why it is so hard to add a check at parse time.
> 
> Breaking buggy setups would not be a concern I think.

It would be possible for xtables but afaics in nft_nat.c case
(nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
registers at runtime.

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Eric Dumazet]

On Wed, 2018-02-14 at 13:30 +0100, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > > syzbot reported a division by 0 bug in the netfilter nat code:
> > > Adding the relevant check at parse time could break existing
> > > setup, moreover we would need to read/write such values atomically
> > > to avoid possible transient negative ranges at update time.
> > 
> > I do not quite follow why it is so hard to add a check at parse time.
> > 
> > Breaking buggy setups would not be a concern I think.
> 
> It would be possible for xtables but afaics in nft_nat.c case
> (nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
> registers at runtime.

I prefer this explanation much more, I suggest we update the changelog
to explain the real reason.

Thanks Florian.

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Pablo Neira Ayuso]

On Wed, Feb 14, 2018 at 01:30:37PM +0100, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > > syzbot reported a division by 0 bug in the netfilter nat code:
> > 
> > > Adding the relevant check at parse time could break existing
> > > setup, moreover we would need to read/write such values atomically
> > > to avoid possible transient negative ranges at update time.
> > 
> > I do not quite follow why it is so hard to add a check at parse time.
> > 
> > Breaking buggy setups would not be a concern I think.
> 
> It would be possible for xtables but afaics in nft_nat.c case
> (nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
> registers at runtime.

Then, restrict this from nft_nat.

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Paolo Abeni]

Hi,

On Wed, 2018-02-14 at 14:51 +0100, Pablo Neira Ayuso wrote:
> On Wed, Feb 14, 2018 at 01:30:37PM +0100, Florian Westphal wrote:
> > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > > > syzbot reported a division by 0 bug in the netfilter nat code:
> > > > Adding the relevant check at parse time could break existing
> > > > setup, moreover we would need to read/write such values atomically
> > > > to avoid possible transient negative ranges at update time.
> > > 
> > > I do not quite follow why it is so hard to add a check at parse time.
> > > 
> > > Breaking buggy setups would not be a concern I think.
> > 
> > It would be possible for xtables but afaics in nft_nat.c case
> > (nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
> > registers at runtime.
> 
> Then, restrict this from nft_nat.

If we move the check in the caller for nft, then need cope individually
with several control paths (nf_nat_setup_info() is used by ~10 modules
if I'm not wrong), I think keeping the check here would be better, do
you have strong opinions against that?

Thanks,

Paolo

Re: [PATCH net v2] netfilter: nat: cope with negative port range

[Pablo Neira Ayuso]

On Wed, Feb 14, 2018 at 04:45:31PM +0100, Paolo Abeni wrote:
> Hi,
> 
> On Wed, 2018-02-14 at 14:51 +0100, Pablo Neira Ayuso wrote:
> > On Wed, Feb 14, 2018 at 01:30:37PM +0100, Florian Westphal wrote:
> > > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > > On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > > > > syzbot reported a division by 0 bug in the netfilter nat code:
> > > > > Adding the relevant check at parse time could break existing
> > > > > setup, moreover we would need to read/write such values atomically
> > > > > to avoid possible transient negative ranges at update time.
> > > > 
> > > > I do not quite follow why it is so hard to add a check at parse time.
> > > > 
> > > > Breaking buggy setups would not be a concern I think.
> > > 
> > > It would be possible for xtables but afaics in nft_nat.c case
> > > (nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
> > > registers at runtime.
> > 
> > Then, restrict this from nft_nat.
> 
> If we move the check in the caller for nft, then need cope individually
> with several control paths (nf_nat_setup_info() is used by ~10 modules
> if I'm not wrong), I think keeping the check here would be better, do
> you have strong opinions against that?

You're right, this is fine.

Thanks for explaining!