[PATCH net 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_rt_gc_expire is a u32, but gc_elasticity has no upper bound, so
writing >= 32 shifts a 32-bit value by >= 32 bits, which is 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

Cap it at 31. 31 and 32 are equivalent anyway: ip6_rt_gc_expire starts
at 30*HZ (~30000 with HZ=1000), and such a value shifted right by 15 or
more is already 0, so val - (val >> rt_elasticity) stops decaying well
before 31. Values above 31 add no new behaviour, they only hit the UB.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv6/route.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 94e5ef1c4f83..6f2efc219f39 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",
@@ -6627,7 +6629,9 @@ static const struct ctl_table ipv6_route_table_template[] = {
 		.data		=	&init_net.ipv6.sysctl.ip6_rt_gc_elasticity,
 		.maxlen		=	sizeof(int),
 		.mode		=	0644,
-		.proc_handler	=	proc_dointvec,
+		.proc_handler	=	proc_dointvec_minmax,
+		.extra1		=	SYSCTL_ZERO,
+		.extra2		=	&ip6_rt_gc_elasticity_max,
 	},
 	{
 		.procname	=	"mtu_expires",
-- 
2.43.0