[PATCH net v2 1/5] ipv6: restrict gc_elasticity to 31

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Every counted IPv6 dst allocation runs ip6_dst_gc() once the number of
live dst goes over gc_thresh (1024 by default). ip6_dst_gc() decays
ip6_rt_gc_expire by gc_elasticity: the smaller gc_elasticity is, the
smaller ip6_rt_gc_expire gets, i.e. the more aggressive the gc.

ip6_dst_gc() reads ip6_rt_gc_expire into a u32, but gc_elasticity has no
bounds at all, so writing >= 32 shifts a 32-bit value by >= 32 bits, and
a negative value gives a negative shift count. Both are UB:

	val = atomic_read(&net->ipv6.ip6_rt_gc_expire);
	atomic_set(&net->ipv6.ip6_rt_gc_expire, val - (val >> rt_elasticity));

  UBSAN: shift-out-of-bounds in net/ipv6/route.c:3369:53
  shift exponent 32 is too large for 32-bit type 'unsigned int'
  CPU: 2 UID: 0 PID: 615 Comm: ping6 Not tainted 7.3.0-rc7+ #364 PREEMPT
  Call Trace:
   <IRQ>
   dump_stack_lvl+0xc1/0xf0
   dump_stack+0x10/0x20
   __ubsan_handle_shift_out_of_bounds+0x1d1/0x390
   ip6_dst_gc.cold+0x15/0x1a
   dst_alloc+0x4e0/0x620
   ip6_rt_cache_alloc+0x189/0x7e0
   __ip6_rt_update_pmtu+0x68e/0x1160
   ip6_update_pmtu+0x30c/0x3a0
   icmpv6_err+0x1fa/0x330
   icmpv6_notify+0x303/0x770
   icmpv6_rcv+0xc12/0x1670
   ip6_protocol_deliver_rcu+0xb4a/0x1520
   ip6_input_finish+0x19c/0x400
   ip6_input+0xd4/0x240
   ipv6_rcv+0x1fd/0x330
   __netif_receive_skb_one_core+0x118/0x1c0
   __netif_receive_skb+0x21/0x150
   process_backlog+0x37e/0x1200
   __napi_poll.constprop.0+0xa5/0x5e0
   net_rx_action+0x4c0/0xf70
   handle_softirqs+0x204/0x9b0
   __do_softirq+0x10/0x18

Make it a u8 and reject writes outside 0..31 with proc_dou8vec_minmax().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
v2: switched to u8 and proc_dou8vec_minmax(), as suggested by Kuniyuki
Iwashima.
---
 include/net/netns/ipv6.h | 2 +-
 net/ipv6/route.c         | 7 +++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
index ffda15318be59..aefce37c04a4f 100644
--- a/include/net/netns/ipv6.h
+++ b/include/net/netns/ipv6.h
@@ -25,11 +25,11 @@ struct netns_sysctl_ipv6 {
 	int ip6_rt_gc_min_interval;
 	int ip6_rt_gc_timeout;
 	int ip6_rt_gc_interval;
-	int ip6_rt_gc_elasticity;
 	int ip6_rt_mtu_expires;
 	int ip6_rt_min_advmss;
 	u32 multipath_hash_fields;
 	u8 multipath_hash_policy;
+	u8 ip6_rt_gc_elasticity;
 
 	__cacheline_group_begin(sysctl_ipv6_flowlabel);
 	u8 flowlabel_consistency;
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 94e5ef1c4f83e..abc8b811cd423 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -6579,6 +6579,8 @@ static int ipv6_sysctl_rtcache_flush(const struct ctl_table *ctl, int write,
 	return 0;
 }
 
+static int ip6_rt_gc_elasticity_max = 31;
+
 static const struct ctl_table ipv6_route_table_template[] = {
 	{
 		.procname	=	"max_size",
@@ -6625,9 +6627,10 @@ static const struct ctl_table ipv6_route_table_template[] = {
 	{
 		.procname	=	"gc_elasticity",
 		.data		=	&init_net.ipv6.sysctl.ip6_rt_gc_elasticity,
-		.maxlen		=	sizeof(int),
+		.maxlen		=	sizeof(u8),
 		.mode		=	0644,
-		.proc_handler	=	proc_dointvec,
+		.proc_handler	=	proc_dou8vec_minmax,
+		.extra2		=	&ip6_rt_gc_elasticity_max,
 	},
 	{
 		.procname	=	"mtu_expires",
-- 
2.43.0