net: dst: fix missing initialization of rt_uncached

xfrm_alloc_dst() followed by xfrm4_dst_destroy(), without a
xfrm4_fill_dst() call in between, causes the following BUG:

BUG: spinlock bad magic on CPU#0, fbxhostapd/732
lock: 0x890b7668, .magic: 890b7668, .owner: <none>/-1, .owner_cpu: 0
CPU: 0 PID: 732 Comm: fbxhostapd Not tainted 6.3.0-rc6-next-20230414-00613-ge8de66369925-dirty #9
Hardware name: Marvell Kirkwood (Flattened Device Tree)
unwind_backtrace from show_stack+0x10/0x14
show_stack from dump_stack_lvl+0x28/0x30
dump_stack_lvl from do_raw_spin_lock+0x20/0x80
do_raw_spin_lock from rt_del_uncached_list+0x30/0x64
rt_del_uncached_list from xfrm4_dst_destroy+0x3c/0xbc
xfrm4_dst_destroy from dst_destroy+0x5c/0xb0
dst_destroy from rcu_process_callbacks+0xc4/0xec
rcu_process_callbacks from __do_softirq+0xb4/0x22c
__do_softirq from call_with_stack+0x1c/0x24
call_with_stack from do_softirq+0x60/0x6c
do_softirq from __local_bh_enable_ip+0xa0/0xcc

Patch "net: dst: Prevent false sharing vs. dst_entry:: __refcnt" moved
rt_uncached and rt_uncached_list fields from rtable struct to dst
struct, so they are more zeroed by memset_after(xdst, 0, u.dst) in
xfrm_alloc_dst().

Note that rt_uncached (list_head) was never properly initialized at
alloc time, but xfrm[46]_dst_destroy() is written in such a way that
it was not an issue thanks to the memset:

if (xdst->u.rt.dst.rt_uncached_list)
rt_del_uncached_list(&xdst->u.rt);

The route code does it the other way around: rt_uncached_list is
assumed to be valid IIF rt_uncached list_head is not empty:

void rt_del_uncached_list(struct rtable *rt)
{
if (!list_empty(&rt->dst.rt_uncached)) {
struct uncached_list *ul = rt->dst.rt_uncached_list;

spin_lock_bh(&ul->lock);
list_del_init(&rt->dst.rt_uncached);
spin_unlock_bh(&ul->lock);
}
}

This patch adds mandatory rt_uncached list_head initialization in
generic dst_init(), and adapt xfrm[46]_dst_destroy logic to match the
rest of the code.

Fixes: d288a162dd1c ("net: dst: Prevent false sharing vs. dst_entry:: __refcnt")
Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/oe-lkp/[e-mail]
Reviewed-by: David Ahern <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
CC: Leon Romanovsky <[e-mail]>
Signed-off-by: Maxime Bizon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/dst.c

···
 #endif
 	dst->lwtstate = NULL;
 	rcuref_init(&dst->__rcuref, initial_ref);
+	INIT_LIST_HEAD(&dst->rt_uncached);
 	dst->__use = 0;
 	dst->lastuse = jiffies;
 	dst->flags = flags;

net/ipv4/route.c

···
 		rt->rt_uses_gateway = 0;
 		rt->rt_gw_family = 0;
 		rt->rt_gw4 = 0;
-		INIT_LIST_HEAD(&rt->dst.rt_uncached);
 
 		rt->dst.output = ip_output;
 		if (flags & RTCF_LOCAL)
···
 			new_rt->rt_gw4 = rt->rt_gw4;
 		else if (rt->rt_gw_family == AF_INET6)
 			new_rt->rt_gw6 = rt->rt_gw6;
-		INIT_LIST_HEAD(&new_rt->dst.rt_uncached);
 
 		new_rt->dst.input = rt->dst.input;
 		new_rt->dst.output = rt->dst.output;
···
 			rt->rt_gw4 = ort->rt_gw4;
 		else if (rt->rt_gw_family == AF_INET6)
 			rt->rt_gw6 = ort->rt_gw6;
-
-		INIT_LIST_HEAD(&rt->dst.rt_uncached);
 	}
 
 	dst_release(dst_orig);

net/ipv4/xfrm4_policy.c

···
 		xdst->u.rt.rt_gw6 = rt->rt_gw6;
 	xdst->u.rt.rt_pmtu = rt->rt_pmtu;
 	xdst->u.rt.rt_mtu_locked = rt->rt_mtu_locked;
-	INIT_LIST_HEAD(&xdst->u.rt.dst.rt_uncached);
 	rt_add_uncached_list(&xdst->u.rt);
 
 	return 0;
···
 	struct xfrm_dst *xdst = (struct xfrm_dst *)dst;
 
 	dst_destroy_metrics_generic(dst);
-	if (xdst->u.rt.dst.rt_uncached_list)
-		rt_del_uncached_list(&xdst->u.rt);
+	rt_del_uncached_list(&xdst->u.rt);
 	xfrm_dst_destroy(xdst);
 }
 

net/ipv6/route.c

···
 static void rt6_info_init(struct rt6_info *rt)
 {
 	memset_after(rt, 0, dst);
-	INIT_LIST_HEAD(&rt->dst.rt_uncached);
 }
 
 /* allocate dst with ip6_dst_ops */

net/ipv6/xfrm6_policy.c

···
 	xdst->u.rt6.rt6i_gateway = rt->rt6i_gateway;
 	xdst->u.rt6.rt6i_dst = rt->rt6i_dst;
 	xdst->u.rt6.rt6i_src = rt->rt6i_src;
-	INIT_LIST_HEAD(&xdst->u.rt6.dst.rt_uncached);
 	rt6_uncached_list_add(&xdst->u.rt6);
 
 	return 0;
···
 	if (likely(xdst->u.rt6.rt6i_idev))
 		in6_dev_put(xdst->u.rt6.rt6i_idev);
 	dst_destroy_metrics_generic(dst);
-	if (xdst->u.rt6.dst.rt_uncached_list)
-		rt6_uncached_list_del(&xdst->u.rt6);
+	rt6_uncached_list_del(&xdst->u.rt6);
 	xfrm_dst_destroy(xdst);
 }