[PATCH net v3 1/1] xfrm: save input state data before secpath resets

From: Zhiling Zou <[email protected]>

xfrm_input() keeps the current state reference through the skb secpath
while it performs final transport processing. Some input paths can reset
the secpath before xfrm_input() is done using data from that state.

Receive callback users such as VTI and XFRM interfaces can scrub packets
that cross network namespaces by resetting the secpath. The
XFRM_MAX_DEPTH error path can also reset the secpath before the final drop
callback reports the current state's protocol. If that drops the last
state reference and the state is concurrently deleted, xfrm_input() can
still dereference the freed state.

Save the state protocol on the stack while the state is still valid, and
use the already saved address family for transport_finish(). A larval
XFRM_STATE_ACQ state has no type, so retain nexthdr as its protocol. This
preserves the existing drop-path fallback while avoiding the post-reset
state dereferences without adding an extra state reference to every
received packet.

Fixes: df3893c176e9 ("vti: Update the ipv4 side to use it's own receive hook.")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Signed-off-by: Zhiling Zou <[email protected]>
---
changes in v3:
- Retain nexthdr for larval XFRM_STATE_ACQ entries, whose x->type is NULL.
- Avoid dereferencing x->type before the existing state validity checks.
- v2 Link: https://lore.kernel.org/all/e59b617d27acb3a38a3523649d52b90f022fa28d.1785288865.git.zhilinz@nebusec.ai/

changes in v2:
- Avoid per-packet xfrm_state refcounting by saving state data on the stack.
- Use the saved protocol and family after secpath resets instead of
  dereferencing x.
- v1 Link: https://lore.kernel.org/all/89be67de93958a00193c8c4bd668733c976e2aa9.1785135576.git.zhilinz@nebusec.ai/

 net/xfrm/xfrm_input.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
index eecab337bd0a7..77b9f4d4568d6 100644
--- a/net/xfrm/xfrm_input.c
+++ b/net/xfrm/xfrm_input.c
@@ -474,6 +474,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 	struct xfrm_state *x = NULL;
 	xfrm_address_t *daddr;
 	u32 mark = skb->mark;
+	u8 xfrm_proto = nexthdr;
 	unsigned int family = AF_UNSPEC;
 	int decaps = 0;
 	int async = 0;
@@ -485,6 +486,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 	if (encap_type < 0 || (xo && (xo->flags & XFRM_GRO || encap_type == 0 ||
 				      encap_type == UDP_ENCAP_ESPINUDP))) {
 		x = xfrm_input_state(skb);
+		xfrm_proto = x->type ? x->type->proto : nexthdr;
 
 		if (unlikely(x->km.state != XFRM_STATE_VALID)) {
 			if (x->km.state == XFRM_STATE_ACQ)
@@ -592,11 +594,13 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 
 		x = xfrm_input_state_lookup(net, mark, daddr, spi, nexthdr, family);
 		if (x == NULL) {
+			xfrm_proto = nexthdr;
 			secpath_reset(skb);
 			XFRM_INC_STATS(net, LINUX_MIB_XFRMINNOSTATES);
 			xfrm_audit_state_notfound(skb, family, spi, seq);
 			goto drop;
 		}
+		xfrm_proto = x->type ? x->type->proto : nexthdr;
 
 		if (unlikely(x->dir && x->dir != XFRM_SA_DIR_IN)) {
 			secpath_reset(skb);
@@ -604,6 +608,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 			xfrm_audit_state_notfound(skb, family, spi, seq);
 			xfrm_state_put(x);
 			x = NULL;
+			xfrm_proto = nexthdr;
 			goto drop;
 		}
 
@@ -728,7 +733,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 	} while (!err);
 
 	rcu_read_lock();
-	err = xfrm_rcv_cb(skb, family, x->type->proto, 0);
+	err = xfrm_rcv_cb(skb, family, xfrm_proto, 0);
 	if (err) {
 		rcu_read_unlock();
 		goto drop;
@@ -753,7 +758,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 			xfrm_gro = xo->flags & XFRM_GRO;
 
 		err = -EAFNOSUPPORT;
-		afinfo = xfrm_state_afinfo_get_rcu(x->props.family);
+		afinfo = xfrm_state_afinfo_get_rcu(family);
 		if (likely(afinfo))
 			err = afinfo->transport_finish(skb, xfrm_gro || async);
 		if (xfrm_gro) {
@@ -776,7 +781,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 drop:
 	if (async)
 		dev_put(dev);
-	xfrm_rcv_cb(skb, family, x && x->type ? x->type->proto : nexthdr, -1);
+	xfrm_rcv_cb(skb, family, xfrm_proto, -1);
 	kfree_skb(skb);
 	return 0;
 }
-- 
2.43.0