[PATCH NET v4 1/1] ipv6: allocate enough headroom in ip6_finish_output2()

When TEE target mirrors traffic to another interface, sk_buff may
not have enough headroom to be processed correctly.
ip_finish_output2() detect this situation for ipv4 and allocates
new skb with enogh headroom. However ipv6 lacks this logic in
ip_finish_output2 and it leads to skb_under_panic:

 skbuff: skb_under_panic: text:ffffffffc0866ad4 len:96 put:24
 head:ffff97be85e31800 data:ffff97be85e317f8 tail:0x58 end:0xc0 dev:gre0
 ------------[ cut here ]------------
 kernel BUG at net/core/skbuff.c:110!
 invalid opcode: 0000 [#1] SMP PTI
 CPU: 2 PID: 393 Comm: kworker/2:2 Tainted: G           OE     5.13.0 #13
 Hardware name: Virtuozzo KVM, BIOS 1.11.0-2.vz7.4 04/01/2014
 Workqueue: ipv6_addrconf addrconf_dad_work
 RIP: 0010:skb_panic+0x48/0x4a
 Call Trace:
  skb_push.cold.111+0x10/0x10
  ipgre_header+0x24/0xf0 [ip_gre]
  neigh_connected_output+0xae/0xf0
  ip6_finish_output2+0x1a8/0x5a0
  ip6_output+0x5c/0x110
  nf_dup_ipv6+0x158/0x1000 [nf_dup_ipv6]
  tee_tg6+0x2e/0x40 [xt_TEE]
  ip6t_do_table+0x294/0x470 [ip6_tables]
  nf_hook_slow+0x44/0xc0
  nf_hook.constprop.34+0x72/0xe0
  ndisc_send_skb+0x20d/0x2e0
  ndisc_send_ns+0xd1/0x210
  addrconf_dad_work+0x3c8/0x540
  process_one_work+0x1d1/0x370
  worker_thread+0x30/0x390
  kthread+0x116/0x130
  ret_from_fork+0x22/0x30

Signed-off-by: Vasily Averin <redacted>
---
 net/ipv6/ip6_output.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index ff4f9eb..25144c7 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c

@@ -60,10 +60,38 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 {
 	struct dst_entry *dst = skb_dst(skb);
 	struct net_device *dev = dst->dev;
+	unsigned int hh_len = LL_RESERVED_SPACE(dev);
+	int delta = hh_len - skb_headroom(skb);
 	const struct in6_addr *nexthop;
 	struct neighbour *neigh;
 	int ret;
 
+	/* Be paranoid, rather than too clever. */
+	if (unlikely(delta > 0) && dev->header_ops) {
+		/* pskb_expand_head() might crash, if skb is shared */
+		if (skb_shared(skb)) {
+			struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
+
+			if (likely(nskb)) {
+				if (skb->sk)
+					skb_set_owner_w(nskb, skb->sk);
+				consume_skb(skb);
+			} else {
+				kfree_skb(skb);
+			}
+			skb = nskb;
+		}
+		if (skb &&
+		    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+			kfree_skb(skb);
+			skb = NULL;
+		}
+		if (!skb) {
+			IP6_INC_STATS(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTDISCARDS);
+			return -ENOMEM;
+		}
+	}
+
 	if (ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr)) {
 		struct inet6_dev *idev = ip6_dst_idev(skb_dst(skb));
 

-- 
1.8.3.1

Dear David,
I've found that you have added v3 version of this patch into netdev-net git.
This version had one mistake: skb_set_owner_w() should set sk not to old skb byt to new nskb.
I've fixed it in v4 version.

Could you please drop bad v3 version and pick up fixed one ?
Should I perhaps submit separate fixup instead?

Thank you,
	Vasily Averin

On 7/13/21 3:01 PM, Vasily Averin wrote:

On 7/18/21 4:44 AM, Vasily Averin wrote:

I've found that you have added v3 version of this patch into netdev-net git.
This version had one mistake: skb_set_owner_w() should set sk not to old skb byt to new nskb.
I've fixed it in v4 version.

Could you please drop bad v3 version and pick up fixed one ?
Should I perhaps submit separate fixup instead?

Patches are not dropped once pushed; send the diff between v3 and v4
with a Fixes tag.

From: Vasily Averin <redacted>
Date: Sun, 18 Jul 2021 13:44:33 +0300

Dear David,
I've found that you have added v3 version of this patch into netdev-net git.
This version had one mistake: skb_set_owner_w() should set sk not to old skb byt to new nskb.
I've fixed it in v4 version.

Could you please drop bad v3 version and pick up fixed one ?
Should I perhaps submit separate fixup instead?

Always submit a fixup in these situations..

Thank you.

skb_set_owner_w() should set sk not to old skb but to new nskb.

Fixes: 5796015fa968("ipv6: allocate enough headroom in ip6_finish_output2()")
Signed-off-by: Vasily Averin <redacted>
---
 net/ipv6/ip6_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 01bea76..e1b9f7a 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c

@@ -74,7 +74,7 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 			if (likely(nskb)) {
 				if (skb->sk)
-					skb_set_owner_w(skb, skb->sk);
+					skb_set_owner_w(nskb, skb->sk);
 				consume_skb(skb);
 			} else {
 				kfree_skb(skb);

-- 
1.8.3.1

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Mon, 19 Jul 2021 10:55:14 +0300 you wrote:

skb_set_owner_w() should set sk not to old skb but to new nskb.

Fixes: 5796015fa968("ipv6: allocate enough headroom in ip6_finish_output2()")
Signed-off-by: Vasily Averin <redacted>
---
 net/ipv6/ip6_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Here is the summary with links:
  - [NET] ipv6: ip6_finish_output2: set sk into newly allocated nskb
    https://git.kernel.org/netdev/net/c/2d85a1b31dde

You are awesome, thank you!
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