Commit 21ec9277 authored by Jiayuan Chen's avatar Jiayuan Chen Committed by Jakub Kicinski
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net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop 



When a standalone IPv6 nexthop object is created with a loopback device
(e.g., "ip -6 nexthop add id 100 dev lo"), fib6_nh_init() misclassifies
it as a reject route. This is because nexthop objects have no destination
prefix (fc_dst=::), causing fib6_is_reject() to match any loopback
nexthop. The reject path skips fib_nh_common_init(), leaving
nhc_pcpu_rth_output unallocated. If an IPv4 route later references this
nexthop, __mkroute_output() dereferences NULL nhc_pcpu_rth_output and
panics.

Simplify the check in fib6_nh_init() to only match explicit reject
routes (RTF_REJECT) instead of using fib6_is_reject(). The loopback
promotion heuristic in fib6_is_reject() is handled separately by
ip6_route_info_create_nh(). After this change, the three cases behave
as follows:

1. Explicit reject route ("ip -6 route add unreachable 2001:db8::/64"):
   RTF_REJECT is set, enters reject path, skips fib_nh_common_init().
   No behavior change.

2. Implicit loopback reject route ("ip -6 route add 2001:db8::/32 dev lo"):
   RTF_REJECT is not set, takes normal path, fib_nh_common_init() is
   called. ip6_route_info_create_nh() still promotes it to reject
   afterward. nhc_pcpu_rth_output is allocated but unused, which is
   harmless.

3. Standalone nexthop object ("ip -6 nexthop add id 100 dev lo"):
   RTF_REJECT is not set, takes normal path, fib_nh_common_init() is
   called. nhc_pcpu_rth_output is properly allocated, fixing the crash
   when IPv4 routes reference this nexthop.

Suggested-by: default avatarIdo Schimmel <idosch@nvidia.com>
Fixes: 493ced1a ("ipv4: Allow routes to use nexthop objects")
Reported-by: default avatar <syzbot+334190e097a98a1b81bb@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/


Signed-off-by: default avatarJiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: default avatarIdo Schimmel <idosch@nvidia.com>
Reviewed-by: default avatarDavid Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260304113817.294966-2-jiayuan.chen@linux.dev


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 168ff39e

net/ipv6/route.c

+3 −5
Original line number Diff line number Diff line
@@ -3583,7 +3583,6 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh, 
	netdevice_tracker *dev_tracker = &fib6_nh->fib_nh_dev_tracker;

	struct net_device *dev = NULL;

	struct inet6_dev *idev = NULL;

	int addr_type;

	int err;



	fib6_nh->fib_nh_family = AF_INET6;
@@ -3625,11 +3624,10 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh, 


	fib6_nh->fib_nh_weight = 1;



	/* We cannot add true routes via loopback here,

	 * they would result in kernel looping; promote them to reject routes

	/* Reset the nexthop device to the loopback device in case of reject

	 * routes.

	 */

	addr_type = ipv6_addr_type(&cfg->fc_dst);

	if (fib6_is_reject(cfg->fc_flags, dev, addr_type)) {

	if (cfg->fc_flags & RTF_REJECT) {

		/* hold loopback dev/idev if we haven't done so. */

		if (dev != net->loopback_dev) {

			if (dev) {