[PATCH 6.1.y] ipmr: drop packets with unknown ingress VIF

[Sasha Levin <sashal@kernel.org>]

From: Zhiling Zou <zhilinz@nebusec.ai>

[ Upstream commit 09280680f2ea060514be2a39ea84209fb6d721d4 ]

A packet can race with VIF publication and make ipmr_find_vif() return
-1 even though the VIF device pointer is already visible. The parent VIF
device check can then pass with an invalid ingress VIF. For (*,G)
entries, this can also let the packet be forwarded back out its ingress
VIF.

Drop packets whose ingress VIF lookup fails in both IPv4 and IPv6
forwarding, before checking the MFC parent device. This prevents
forwarding with an unknown ingress VIF and avoids the (*,G) hairpin.

Without the early check, the negative index can reach the IPv4 offload
lookup only for an skb marked for L3 forwarding offload, currently done
by mlxsw. The race is rare; the supplied PoC reports only the UBSAN
negative-index diagnostic.

Fixes: 4eadb88244d1 ("ipmr: do not acquire mrt_lock while calling ip_mr_forward()")
Fixes: 9b1c21d898fd ("ip6mr: do not acquire mrt_lock while calling ip6_mr_forward()")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Link: https://lore.kernel.org/netdev/b966eb102e98f299e45be59ada033a8766e4c6df.1791899747.git.zhilinz@nebusec.ai/
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Suggested-by: Naman Gulati <namangulati@google.com>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Naman Gulati <namangulati@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/5271fcb94a4d9e697ed64a348e92ea9e74faab1c.1792608753.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ sashal: Adjusted IPv6 context to preserve wrong_if++ for the unsigned long counter instead of atomic_long_inc(). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv4/ipmr.c  | 3 ++-
 net/ipv6/ip6mr.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 948993a4f4157..452aa813d6017 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1976,7 +1976,8 @@ static void ip_mr_forward(struct net *net, struct mr_table *mrt,
 	}
 
 	/* Wrong interface: drop packet and (maybe) send PIM assert. */
-	if (rcu_access_pointer(mrt->vif_table[vif].dev) != dev) {
+	if (true_vifi < 0 ||
+	    rcu_access_pointer(mrt->vif_table[vif].dev) != dev) {
 		if (rt_is_output_route(skb_rtable(skb))) {
 			/* It is our own packet, looped back.
 			 * Very complicated situation...
diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index b677f48179c87..c7ff2d49253a9 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -2159,7 +2159,8 @@ static void ip6_mr_forward(struct net *net, struct mr_table *mrt,
 	/*
 	 * Wrong interface: drop packet and (maybe) send PIM assert.
 	 */
-	if (rcu_access_pointer(mrt->vif_table[vif].dev) != dev) {
+	if (true_vifi < 0 ||
+	    rcu_access_pointer(mrt->vif_table[vif].dev) != dev) {
 		c->_c.mfc_un.res.wrong_if++;
 
 		if (true_vifi >= 0 && mrt->mroute_do_assert &&
-- 
2.53.0