[PATCH v2 net 4/4] vlan: ensure sufficient headroom in vlan_dev_hard_header()

Callers that only reserve ETH_HLEN or less (such as llc_alloc_frame()),
or skbs allocated before dynamic device/headroom changes (e.g. toggling
VLAN_FLAG_REORDER_HDR or bonding/team switching slaves), can reach
vlan_dev_hard_header() with insufficient headroom and trigger
skb_under_panic().

Use skb_cow_head() in vlan_dev_hard_header() when VLAN_FLAG_REORDER_HDR
is not set to ensure sufficient headroom for the VLAN header(s) and the
underlying device hard header.

Use READ_ONCE() to read dev->hard_header_len and dev->needed_headroom as
they can be updated concurrently under RTNL (e.g. in
vlan_transfer_features()) while vlan_dev_hard_header() runs locklessly on
the transmit path. Also avoid LL_RESERVED_SPACE(dev) here so that the
extra HH_DATA_MOD alignment padding does not trigger unnecessary
pskb_expand_head() reallocations on inner stacked VLAN devices after the
outer VLAN header has been pushed.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Zixuan Chai <redacted>
Closes: https://lore.kernel.org/netdev/cover.1789987105.git.petalzu987@gmail.com/ (local)
Link: https://lore.kernel.org/netdev/179022851638.2160803.1808206741379444999@kernel.org/ (local)
Cc: Hangbin Liu <redacted>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/8021q/vlan_dev.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 2859cbac3f266b7c4e3f44f41280d33ab69c5270..c949c6a829456c2f75d6c514b35a85ff64c493d8 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c

@@ -55,6 +55,11 @@ static int vlan_dev_hard_header(struct sk_buff *skb, struct net_device *dev,
 	int rc;
 
 	if (!(vlan->flags & VLAN_FLAG_REORDER_HDR)) {
+		unsigned int hlen = READ_ONCE(dev->hard_header_len) +
+				    READ_ONCE(dev->needed_headroom);
+
+		if (skb_cow_head(skb, hlen) < 0)
+			return -ENOMEM;
 		vhdr = skb_push(skb, VLAN_HLEN);
 
 		vlan_tci = vlan->vlan_id;

-- 
2.56.0.rc1.310.g51773c2048-goog